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Back to Quadratic Equations
JEE Main 2020
Quadratic Equations
Quadratic Equation and Inequalities
Medium

Question

Let p(x) be a quadratic polynomial such that p(0)=1. If p(x) leaves remainder 4 when divided by x- 1 and it leaves remainder 6 when divided by x + 1; then :

Options

Solution

Key Concepts and Formulas

  • Quadratic Polynomial: A polynomial of degree 2, generally expressed as p(x)=ax2+bx+cp(x) = ax^2 + bx + c, where a,b,ca, b, c are constants and a0a \neq 0.
  • Remainder Theorem: If a polynomial p(x)p(x) is divided by (xk)(x - k), the remainder is p(k)p(k).
  • System of Linear Equations: A set of two or more linear equations containing two or more variables.

Step-by-Step Solution

Step 1: Determine the Constant Term (cc)

We are given that p(0)=1p(0) = 1. This means when we substitute x=0x = 0 into the quadratic equation, the result is 1. We can use this to find the value of cc. Since p(x)=ax2+bx+cp(x) = ax^2 + bx + c, we have: p(0)=a(0)2+b(0)+c=1p(0) = a(0)^2 + b(0) + c = 1 0+0+c=10 + 0 + c = 1 c=1c = 1 Now we know that p(x)=ax2+bx+1p(x) = ax^2 + bx + 1.

Step 2: Apply the Remainder Theorem for Division by (x1)(x - 1)

We are given that p(x)p(x) leaves a remainder of 4 when divided by (x1)(x - 1). By the Remainder Theorem, this means p(1)=4p(1) = 4. Substituting x=1x = 1 into our expression for p(x)p(x), we get: p(1)=a(1)2+b(1)+1=4p(1) = a(1)^2 + b(1) + 1 = 4 a+b+1=4a + b + 1 = 4 a+b=3a + b = 3 This gives us our first equation relating aa and bb: a+b=3a + b = 3.

Step 3: Apply the Remainder Theorem for Division by (x+1)(x + 1)

We are given that p(x)p(x) leaves a remainder of 6 when divided by (x+1)(x + 1). By the Remainder Theorem, this means p(1)=6p(-1) = 6. Substituting x=1x = -1 into our expression for p(x)p(x), we get: p(1)=a(1)2+b(1)+1=6p(-1) = a(-1)^2 + b(-1) + 1 = 6 ab+1=6a - b + 1 = 6 ab=5a - b = 5 This gives us our second equation relating aa and bb: ab=5a - b = 5.

Step 4: Solve the System of Linear Equations

We now have a system of two linear equations: a+b=3a + b = 3 ab=5a - b = 5 We can solve this system by adding the two equations to eliminate bb: (a+b)+(ab)=3+5(a + b) + (a - b) = 3 + 5 2a=82a = 8 a=4a = 4 Now substitute the value of aa back into one of the equations to solve for bb. Using the first equation: 4+b=34 + b = 3 b=1b = -1

Step 5: Construct the Complete Quadratic Polynomial

We have found a=4a = 4, b=1b = -1, and c=1c = 1. Therefore, the quadratic polynomial is: p(x)=4x2x+1p(x) = 4x^2 - x + 1

Step 6: Evaluate the Given Options

Now we need to find which of the options is correct by substituting the given values into the polynomial.

  • Option (A): p(2)=11p(2) = 11 p(2)=4(2)22+1=4(4)2+1=162+1=15p(2) = 4(2)^2 - 2 + 1 = 4(4) - 2 + 1 = 16 - 2 + 1 = 15 Since p(2)=1511p(2) = 15 \neq 11, option (A) is incorrect.

  • Option (B): p(2)=19p(2) = 19 Since p(2)=1519p(2) = 15 \neq 19, option (B) is incorrect.

  • Option (C): p(2)=19p(-2) = 19 p(2)=4(2)2(2)+1=4(4)+2+1=16+2+1=19p(-2) = 4(-2)^2 - (-2) + 1 = 4(4) + 2 + 1 = 16 + 2 + 1 = 19 Since p(2)=19p(-2) = 19, option (C) is correct.

  • Option (D): p(2)=11p(-2) = 11 Since p(2)=1911p(-2) = 19 \neq 11, option (D) is incorrect.

Common Mistakes & Tips

  • Sign Errors: Be careful with signs when applying the Remainder Theorem, especially when dividing by (x+k)(x + k). Remember to substitute x=kx = -k.
  • Solving Linear Equations: Double-check your algebra when solving the system of equations to avoid errors in calculating aa and bb.

Summary

By applying the Remainder Theorem and solving the system of linear equations, we found the quadratic polynomial to be p(x)=4x2x+1p(x) = 4x^2 - x + 1. Evaluating the given options, we found that p(2)=19p(-2) = 19, which corresponds to option (C).

The final answer is \boxed{19}, which corresponds to option (C).

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