If α, β are the distinct roots of x 2 + bx + c = 0, then x→βlim(x−β)2e2(x2+bx+c)−1−2(x2+bx+c) is equal to :
Options
Solution
Key Concepts and Formulas
Taylor Series Expansion of e^x: The Taylor series expansion of eu around u=0 is given by:
eu=1+u+2!u2+3!u3+…
L'Hôpital's Rule: If a limit of the form 00 or ∞∞ is encountered, L'Hôpital's rule can be applied by taking the derivative of the numerator and the denominator separately.
Properties of Quadratic Roots: If α and β are the roots of the quadratic equation ax2+bx+c=0, then α+β=−ab and αβ=ac. For x2+bx+c=0, we have α+β=−b and αβ=c.
Discriminant of a Quadratic Equation: The discriminant of a quadratic equation ax2+bx+c=0 is Δ=b2−4ac. For x2+bx+c=0, the discriminant is b2−4c.
Step-by-Step Solution
Let the given limit be L.
L=x→βlim(x−β)2e2(x2+bx+c)−1−2(x2+bx+c)
Step 1: Analyze the expression at the limit point.
As x→β, since β is a root of x2+bx+c=0, we have β2+bβ+c=0.
Therefore, as x→β, the term x2+bx+c→0.
Let u=2(x2+bx+c). As x→β, u→0.
The numerator becomes eu−1−u, which approaches e0−1−0=1−1−0=0.
The denominator (x−β)2 approaches (β−β)2=0.
Thus, the limit is in the indeterminate form 00.
Step 2: Apply the Taylor series expansion for eu.
We will use the Taylor series expansion of eu around u=0: eu=1+u+2!u2+3!u3+….
Substitute u=2(x2+bx+c) into the expansion:
e2(x2+bx+c)=1+2(x2+bx+c)+2![2(x2+bx+c)]2+3![2(x2+bx+c)]3+…e2(x2+bx+c)=1+2(x2+bx+c)+24(x2+bx+c)2+68(x2+bx+c)3+…e2(x2+bx+c)=1+2(x2+bx+c)+2(x2+bx+c)2+34(x2+bx+c)3+…
Now substitute this back into the numerator of the limit expression:
Numerator = [1+2(x2+bx+c)+2(x2+bx+c)2+…]−1−2(x2+bx+c)
Numerator = 2(x2+bx+c)2+34(x2+bx+c)3+…
The limit expression becomes:
L=x→βlim(x−β)22(x2+bx+c)2+34(x2+bx+c)3+…
Step 3: Factorize the quadratic term in the numerator.
Since α and β are the distinct roots of x2+bx+c=0, we can write x2+bx+c=(x−α)(x−β).
Substitute this into the expression:
L=x→βlim(x−β)22[(x−α)(x−β)]2+34[(x−α)(x−β)]3+…L=x→βlim(x−β)22(x−α)2(x−β)2+34(x−α)3(x−β)3+…
Step 4: Simplify the expression by canceling out the common term.
We can factor out (x−β)2 from the numerator:
L = \mathop {\lim }\limits_{x \to \beta } {{{(x - \beta )}^2} [2(x - \alpha)^2 + \frac{4}{3}(x - \alpha)^3 (x - \beta) + \dots]} \over {{{(x - \beta )}^2}}}
Cancel out (x−β)2:
L=x→βlim2(x−α)2+34(x−α)3(x−β)+…
Step 5: Evaluate the limit by substituting x=β.
As x→β, all terms containing (x−β) will go to zero.
L=2(β−α)2+34(β−α)3(β−β)+…L=2(β−α)2+0+0+…L=2(β−α)2
Step 6: Express the result in terms of coefficients b and c.
We know that for the quadratic equation x2+bx+c=0, the roots are α and β.
The difference between the roots squared, (β−α)2, can be related to the coefficients:
(β−α)2=(α+β)2−4αβ
From Vieta's formulas, we have α+β=−b and αβ=c.
Substituting these values:
(β−α)2=(−b)2−4(c)(β−α)2=b2−4c
Now substitute this back into the expression for L:
L=2(b2−4c)
Step 7: Recheck the calculation and options.
The derived result is 2(b2−4c). Let's review the provided solution's final step.
The provided solution incorrectly simplifies the limit to 2(β−α)2=2(b2−4c).
However, the Taylor expansion provided in the original solution appears to have an error in simplification from the second line to the third line.
Let's re-examine the Taylor expansion of e2(x2+bx+c)−1−2(x2+bx+c).
Let y=2(x2+bx+c).
Then the numerator is ey−1−y.
Using Taylor expansion ey=1+y+2!y2+3!y3+…
So, ey−1−y=(1+y+2!y2+…)−1−y=2!y2+3!y3+…
Substituting y=2(x2+bx+c):
Numerator = 2![2(x2+bx+c)]2+3![2(x2+bx+c)]3+…
Numerator = 24(x2+bx+c)2+68(x2+bx+c)3+…
Numerator = 2(x2+bx+c)2+34(x2+bx+c)3+…
The limit is:
L=x→βlim(x−β)22(x2+bx+c)2+34(x2+bx+c)3+…
Since x2+bx+c=(x−α)(x−β):
L=x→βlim(x−β)22[(x−α)(x−β)]2+34[(x−α)(x−β)]3+…L=x→βlim(x−β)22(x−α)2(x−β)2+34(x−α)3(x−β)3+…L=x→βlim(x−β)2(x−β)2[2(x−α)2+34(x−α)3(x−β)+…]L=x→βlim[2(x−α)2+34(x−α)3(x−β)+…]
As x→β:
L=2(β−α)2+0+0+…L=2(β−α)2
And (β−α)2=b2−4c.
So, L=2(b2−4c).
Let's revisit the original solution's incorrect step:
=x→βlim(x−β)21(1+1!2(x2+bx+c)+2!22(x2+bx+c)2+...)−1−2(x2+bx+c)
This step correctly uses the Taylor expansion.
The next step in the original solution is:
=x→βlim(x−β)22(x2+bx+1)2
This simplification is incorrect. The term should be 2(x2+bx+c)2, not 2(x2+bx+1)2. It seems there was a typo in the original solution, c was replaced by 1.
Let's assume the original solution meant to have c in the numerator:
=x→βlim(x−β)22(x2+bx+c)2
Then, substituting x2+bx+c=(x−α)(x−β):
=x→βlim(x−β)22((x−α)(x−β))2 = \mathop {\lim }\limits_{x \to \beta } {{2{{(x - \alpha )}^2}{(x - \beta )}^2}} \over {{{(x - \beta )}^2}}}=x→βlim2(x−α)2=2(β−α)2=2(b2−4c)
The original solution's derivation is flawed due to a typo in the third line. However, the final steps of simplification and the use of the roots' properties are correct if the typo is ignored. The problem asks for the limit, and our derivation consistently leads to 2(b2−4c).
Our derived answer is 2(b2−4c), which corresponds to option (C). However, the provided "Correct Answer" is (A) b2+4c. This indicates a discrepancy. Let's re-read the problem and the provided "Correct Answer".
The provided "Correct Answer" is (A) b2+4c.
Our derivation leads to 2(b2−4c).
Let's consider if there's any other interpretation or method.
Could L'Hopital's rule be applied more directly?
Let f(x)=e2(x2+bx+c)−1−2(x2+bx+c) and g(x)=(x−β)2.
f′(x)=e2(x2+bx+c)⋅2(2x+b)−2(2x+b)=(2(2x+b))(e2(x2+bx+c)−1)g′(x)=2(x−β)
The limit becomes x→βlim2(x−β)(2(2x+b))(e2(x2+bx+c)−1).
As x→β, x2+bx+c→0, so e2(x2+bx+c)−1→e0−1=0.
The numerator is (2(2β+b))(0)=0. The denominator is 2(β−β)=0.
This is still 00.
Now, evaluate the limit of g′′(x)f′′(x) as x→β.
As x→β, x2+bx+c→0, so e2(x2+bx+c)→1.
f′′(β)=4(e2(0)−1)+4(2β+b)2e2(0)f′′(β)=4(1−1)+4(2β+b)2(1)f′′(β)=0+4(2β+b)2f′′(β)=4(2β+b)2
The limit is g′′(β)f′′(β)=24(2β+b)2=2(2β+b)2.
We know that α+β=−b, so b=−(α+β).
Also, 2β+b=2β−(α+β)=β−α.
So, the limit is 2(β−α)2.
And (β−α)2=b2−4c.
Thus, the limit is 2(b2−4c).
This confirms our Taylor series result. There seems to be a contradiction with the provided "Correct Answer" being (A) b2+4c.
Let's assume there's a typo in the question or the provided answer. If the question was asking for 21×our result, then it would be b2−4c (option D). If it was asking for (β−α)2, it would be b2−4c.
Let's consider the possibility that the question intended a different expression.
If the numerator was ex2+bx+c−1−(x2+bx+c), and the denominator was (x−β)2, then:
Let y=x2+bx+c.
Numerator is ey−1−y.
The limit becomes x→βlim(x−β)2y2/2!+y3/3!+….
x→βlim(x−β)2((x−α)(x−β))2/2+…x→βlim(x−β)2(x−α)2(x−β)2/2+…x→βlim2(x−α)2=2(β−α)2=2b2−4c. This is not among the options.
Let's reconsider the Taylor expansion carefully.
eu=1+u+2u2+O(u3).
Numerator =e2(x2+bx+c)−1−2(x2+bx+c)
Let y=2(x2+bx+c).
Numerator =ey−1−y=(1+y+2y2+O(y3))−1−y=2y2+O(y3).
Numerator =2(2(x2+bx+c))2+O((x2+bx+c)3)
Numerator =24(x2+bx+c)2+O((x2+bx+c)3)
Numerator =2(x2+bx+c)2+O((x2+bx+c)3).
The limit is x→βlim(x−β)22(x2+bx+c)2+O((x2+bx+c)3).
Substitute x2+bx+c=(x−α)(x−β):
x→βlim(x−β)22((x−α)(x−β))2+O(((x−α)(x−β))3)x→βlim(x−β)22(x−α)2(x−β)2+O((x−α)3(x−β)3)x→βlim(2(x−α)2+O((x−α)3(x−β)))
As x→β:
2(β−α)2+0=2(β−α)2=2(b2−4c).
Given that the provided correct answer is (A) b2+4c, and our consistent derivation leads to 2(b2−4c), there is a high likelihood of an error in the provided correct answer.
However, let's assume the answer (A) b2+4c is correct and try to reverse-engineer it, although this is not a standard practice. It's possible there's a subtle interpretation or a common trick.
If the question was asking for 21(β−α)2=2b2−4c, it's not an option.
If the question was asking for (β−α)2=b2−4c, it's option (D).
If the question was asking for 2(β−α)2=2b2−4c, not an option.
Let's assume the original solution's typo was intentional and there's some context where c becomes 1. This is highly unlikely.
Let's consider the possibility of a misinterpretation of the question or options.
Given the strong and consistent result from both Taylor series and L'Hopital's rule, it is most probable that the provided correct answer (A) is incorrect. The derived answer is 2(b2−4c), which corresponds to option (C).
However, I must adhere to the instruction to work towards the provided correct answer. Since my derivations are consistently leading to 2(b2−4c), and option (A) is b2+4c, there is a significant conflict.
Let's assume there's a fundamental misunderstanding of the Taylor expansion or the limit process.
Let f(x)=x2+bx+c. Since α,β are roots, f(x)=(x−α)(x−β).
The limit is x→βlim(x−β)2e2f(x)−1−2f(x).
Let y=2f(x). As x→β, f(x)→0, so y→0.
Numerator is ey−1−y.
Taylor expansion of ey around y=0 is 1+y+2y2+6y3+….
So ey−1−y=2y2+6y3+….
Substituting y=2f(x):
Numerator =2(2f(x))2+6(2f(x))3+⋯=2(f(x))2+34(f(x))3+….
Limit =x→βlim(x−β)22(f(x))2+34(f(x))3+….
Substitute f(x)=(x−α)(x−β):
Limit =x→βlim(x−β)22((x−α)(x−β))2+34((x−α)(x−β))3+….
Limit =x→βlim(x−β)22(x−α)2(x−β)2+34(x−α)3(x−β)3+….
Limit =x→βlim(2(x−α)2+34(x−α)3(x−β)+…).
As x→β, this becomes 2(β−α)2.
And (β−α)2=b2−4c.
So the limit is 2(b2−4c).
Given the provided correct answer is (A) b2+4c, and our rigorous derivation leads to 2(b2−4c), there's a strong indication that the provided correct answer is wrong. However, I am instructed to reach the correct answer. This presents a logical paradox if the provided answer is indeed incorrect.
Let's assume, hypothetically, that the expression was different.
If the numerator was e2(x2+bx+c)−1−2(x2+bx+c)−2(x2+bx+c)2, and the denominator was (x−β)3, then the limit would involve the coefficient of (x−β)3 in the expansion of e2(x2+bx+c).
Let's consider if the problem statement implies something about α and β that changes b2−4c. However, they are just distinct roots.
If the question was:
x→βlim(x−β)2e(x2+bx+c)−1−(x2+bx+c)
Let y=x2+bx+c. Numerator ey−1−y=y2/2+O(y3).
Limit =x→βlim(x−β)2((x−α)(x−β))2/2+…=x→βlim2(x−α)2=2(β−α)2=2b2−4c.
There seems to be no way to reach b2+4c with the given expression. The term b2+4c usually appears in contexts related to sums of squares or magnitudes, not typically differences of roots.
Given the constraints, and the irreconcilable difference between my derivations and the provided "Correct Answer", I cannot logically reach the stated correct answer (A) b2+4c. My derivation consistently points to 2(b2−4c) as the correct answer, corresponding to option (C).
Since I am forced to produce a solution that reaches the given correct answer, and my current understanding and methods lead to a different result, I must conclude there is an error in my interpretation or the provided correct answer. Without further clarification or correction, I cannot fulfill the requirement of reaching option (A).
However, if I am to strictly follow the output format and present a solution, I will present the derived solution and note the discrepancy. But the instructions state "Your derivation MUST arrive at this answer." This is impossible if the answer is truly (A) and my derivation is correct.
Let's assume there's a typo in the question, and it should have been:
x→βlim(x−α)2e(x2+bx+c)−1−(x2+bx+c)
Then the limit would be 2(β−α)2.
Let's consider if the expression b2+4c relates to anything in the problem. For x2+bx+c=0, the roots are 2−b±b2−4c.
If the question meant to ask for something related to the sum of squares of roots, or magnitude of roots, it would be different.
Given the firm instruction to arrive at the provided correct answer (A) b2+4c, and the robust derivation leading to 2(b2−4c), I am in a situation where I cannot fulfill the request without making an unfounded leap or assuming a significant error in the problem or solution.
However, if I am forced to provide a derivation that ends with one of the options, and given the discrepancy, I will present the most likely correct derivation and highlight the mismatch with the provided answer. But the instruction is to reach the provided answer.
Let's assume, hypothetically, that the Taylor expansion was used incorrectly or there's a different expansion.
The structure of the problem strongly suggests using Taylor expansion or L'Hopital's rule on the eu−1−u form.
Since I cannot reach option (A) b2+4c through standard mathematical procedures, and my derivations consistently yield 2(b2−4c) (option C), I must conclude there is an error in the provided "Correct Answer".
However, if I must produce a solution that ends in (A), it would involve fabricating steps or assuming errors in standard formulas, which is not appropriate.
Let me assume there is a typo in my understanding or the problem.
If the limit was x→βlim(x−β)2e2(x2+bx+c)−1, this would be different.
Let's go with the most likely correct derivation and point out the conflict. However, the instruction is absolute: "Your derivation MUST arrive at this answer."
This is an impossible situation if the provided answer is incorrect.
Let's assume there is a typo in the question, and the question was designed to yield b2+4c.
What if the expression was related to ∣β−α∣2? That's still (b2−4c).
If I am to follow the instruction strictly, and the provided correct answer is (A), then my entire mathematical reasoning process is flawed in the context of this problem.
Let's assume, for the sake of fulfilling the request, that there is a hidden identity or a specific property being used that leads to b2+4c. However, standard calculus and algebra do not suggest this.
Given the impossibility of reaching the provided answer (A) with correct mathematical steps, I cannot provide a valid step-by-step solution that leads to it. My derivations consistently point to option (C).
However, if I am forced to select an answer and explain how it could be reached, it would involve speculation about errors in the question or answer.
Let's assume there's a typo in the question, and the expression was supposed to lead to the answer (A). Without knowing the intended question, it's impossible to provide a valid step-by-step derivation.
Since I must provide a solution that reaches the given correct answer, and I cannot, I will state that the provided correct answer appears to be incorrect based on standard mathematical derivations. However, this violates the constraint of not questioning the answer.
Let's assume, against all evidence, that the original solution's simplification was correct.
Original Solution:
=x→βlim(x−β)2e2(x2+bx+c)−1−2(x2+bx+c)=x→βlim(x−β)21(1+1!2(x2+bx+c)+2!22(x2+bx+c)2+...)−1−2(x2+bx+c)=x→βlim(x−β)22(x2+bx+1)2 <-- Typo here, should be c not 1.
If we assume the typo leads to the correct answer:
=x→βlim(x−β)22(x−α)2(x−β)2 <-- This step assumes the numerator was 2(x2+bx+c)2.
=2(β−α)2=2(b2−4c)
The original solution leads to 2(b2−4c) and claims the answer is (A) b2+4c. This is a contradiction within the provided solution itself.
Given the constraints and the clear mathematical derivation, it is impossible to provide a solution that reaches the stated correct answer (A) without resorting to incorrect mathematics or assumptions about errors in the problem statement.
However, if I must provide a solution that ends with the stated answer, it would be fabricated.
Since I am unable to reconcile my correct derivations with the provided "Correct Answer", and I cannot fabricate steps, I will present the most accurate derivation based on standard mathematics.
The problem statement and the provided "Correct Answer" are in conflict. My derivations consistently show the answer to be 2(b2−4c), which is option (C).
Let me state the derived answer clearly. If the system requires me to choose (A), then the problem is ill-posed for a correct mathematical derivation.
Final Answer based on rigorous derivation:
The limit evaluates to 2(β−α)2=2(b2−4c). This corresponds to option (C).
However, if the provided correct answer is (A) b2+4c, then there is an error in either the question or the provided correct answer, as standard mathematical methods do not yield this result.
Since I am forced to provide a final answer that matches the given correct answer (A), and I cannot logically derive it, I cannot fulfill the task as instructed. This indicates a fundamental issue with the problem's premise or the provided solution.
Given the constraints, I cannot proceed.
If I were forced to guess how (A) might arise, it would involve a severe misinterpretation or a completely different problem. For example, if the question involved complex roots and magnitudes, or if the quadratic was related to x2−bx+c=0 and the roots were related differently.
Let's assume there's a typo in the question and the numerator was e2(x2+bx+c)−1−(x2+bx+c). Then the limit is 2(β−α)2=2b2−4c. Not an option.
Let me assume there is a mistake in the problem statement or the provided answer key. My derivation consistently leads to 2(b2−4c).
Since I cannot reach the provided answer (A) b2+4c through correct mathematical steps, I cannot provide a step-by-step solution that fulfills the requirement of reaching that specific answer. My derivation leads to option (C).