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JEE Main 2020
Quadratic Equations
Quadratic Equation and Inequalities
Medium

Question

If α,β\alpha, \beta are the roots of the equation x2(5+3log355log53)x+3(3(log35)135(log53)231)=0x^{2}-\left(5+3^{\sqrt{\log _{3} 5}}-5^{\sqrt{\log _{5} 3}}\right)x+3\left(3^{\left(\log _{3} 5\right)^{\frac{1}{3}}}-5^{\left(\log _{5} 3\right)^{\frac{2}{3}}}-1\right)=0, then the equation, whose roots are α+1β\alpha+\frac{1}{\beta} and β+1α\beta+\frac{1}{\alpha}, is :

Options

Solution

Key Concepts and Formulas

  • For a quadratic equation ax2+bx+c=0ax^2 + bx + c = 0, the sum of roots is ba-\frac{b}{a} and the product of roots is ca\frac{c}{a}.
  • Change of base formula for logarithms: logab=logcblogca\log_a b = \frac{\log_c b}{\log_c a}. In particular, logab=1logba\log_a b = \frac{1}{\log_b a}.
  • Exponential identity: alogab=ba^{\log_a b} = b and (am)n=amn(a^m)^n = a^{mn}.

Step-by-Step Solution

1. Simplify the Given Quadratic Equation

The given equation is: x2(5+3log355log53)x+3(3(log35)135(log53)231)=0x^{2}-\left(5+3^{\sqrt{\log _{3} 5}}-5^{\sqrt{\log _{5} 3}}\right)x+3\left(3^{\left(\log _{3} 5\right)^{\frac{1}{3}}}-5^{\left(\log _{5} 3\right)^{\frac{2}{3}}}-1\right)=0 We aim to simplify the coefficients of this equation using properties of logarithms and exponents.

2. Simplify the Coefficient of x

The coefficient of xx is 5+3log355log535+3^{\sqrt{\log _{3} 5}}-5^{\sqrt{\log _{5} 3}}. Let k=log35k = \log_3 5. Then log53=1log35=1k\log_5 3 = \frac{1}{\log_3 5} = \frac{1}{k}. Consider the term 3log355log53=3k51/k3^{\sqrt{\log _{3} 5}}-5^{\sqrt{\log _{5} 3}} = 3^{\sqrt{k}} - 5^{\sqrt{1/k}}. Since 5=3log35=3k5 = 3^{\log_3 5} = 3^k, we can write 51/k=(3k)1/k=3k1/k=3k1k=3k5^{\sqrt{1/k}} = (3^k)^{\sqrt{1/k}} = 3^{k \cdot \sqrt{1/k}} = 3^{k \cdot \frac{1}{\sqrt{k}}} = 3^{\sqrt{k}}. Therefore, 3log355log53=3k3k=03^{\sqrt{\log _{3} 5}}-5^{\sqrt{\log _{5} 3}} = 3^{\sqrt{k}} - 3^{\sqrt{k}} = 0. The coefficient of xx simplifies to 5+0=55 + 0 = 5.

3. Simplify the Constant Term

The constant term is 3(3(log35)135(log53)231)3\left(3^{\left(\log _{3} 5\right)^{\frac{1}{3}}}-5^{\left(\log _{5} 3\right)^{\frac{2}{3}}}-1\right). Let m=log35m = \log_3 5. Then log53=1m\log_5 3 = \frac{1}{m}. Consider the term 3(log35)135(log53)23=3m1/35(1/m)2/33^{\left(\log _{3} 5\right)^{\frac{1}{3}}}-5^{\left(\log _{5} 3\right)^{\frac{2}{3}}} = 3^{m^{1/3}} - 5^{(1/m)^{2/3}}. Substitute 5=3m5 = 3^m, then 5(1/m)2/3=(3m)(1/m)2/3=3m(1/m)2/3=3m(1/m2/3)=3m12/3=3m1/35^{(1/m)^{2/3}} = (3^m)^{(1/m)^{2/3}} = 3^{m \cdot (1/m)^{2/3}} = 3^{m \cdot (1/m^{2/3})} = 3^{m^{1 - 2/3}} = 3^{m^{1/3}}. So, 3(log35)135(log53)23=3m1/33m1/3=03^{\left(\log _{3} 5\right)^{\frac{1}{3}}}-5^{\left(\log _{5} 3\right)^{\frac{2}{3}}} = 3^{m^{1/3}} - 3^{m^{1/3}} = 0. The expression inside the parenthesis of the constant term becomes 01=10 - 1 = -1. The constant term simplifies to 3×(1)=33 \times (-1) = -3.

4. Write the Simplified Quadratic Equation

The original quadratic equation simplifies to: x25x3=0x^2 - 5x - 3 = 0

5. Determine the Sum and Product of Original Roots

For the equation x25x3=0x^2 - 5x - 3 = 0, let the roots be α\alpha and β\beta.

  • Sum of roots: α+β=(5)/1=5\alpha + \beta = -(-5)/1 = 5.
  • Product of roots: αβ=3/1=3\alpha\beta = -3/1 = -3.

6. Define the New Roots

The new roots are given as p=α+1βp = \alpha + \frac{1}{\beta} and q=β+1αq = \beta + \frac{1}{\alpha}.

7. Calculate the Sum of the New Roots

p+q=(α+1β)+(β+1α)p+q = \left(\alpha + \frac{1}{\beta}\right) + \left(\beta + \frac{1}{\alpha}\right) p+q=(α+β)+(1α+1β)p+q = (\alpha + \beta) + \left(\frac{1}{\alpha} + \frac{1}{\beta}\right) To find 1α+1β\frac{1}{\alpha} + \frac{1}{\beta}, we combine the fractions: 1α+1β=β+ααβ\frac{1}{\alpha} + \frac{1}{\beta} = \frac{\beta + \alpha}{\alpha\beta} Substitute the values of α+β\alpha + \beta and αβ\alpha\beta: 1α+1β=53=53\frac{1}{\alpha} + \frac{1}{\beta} = \frac{5}{-3} = -\frac{5}{3} Now, substitute this back into the sum of new roots: p+q=(α+β)+α+βαβ=5+(53)=1553=103p+q = (\alpha + \beta) + \frac{\alpha + \beta}{\alpha\beta} = 5 + \left(-\frac{5}{3}\right) = \frac{15 - 5}{3} = \frac{10}{3}

8. Calculate the Product of the New Roots

pq=(α+1β)(β+1α)pq = \left(\alpha + \frac{1}{\beta}\right)\left(\beta + \frac{1}{\alpha}\right) Expand the product: pq=αβ+α(1α)+(1β)β+(1β)(1α)pq = \alpha\beta + \alpha\left(\frac{1}{\alpha}\right) + \left(\frac{1}{\beta}\right)\beta + \left(\frac{1}{\beta}\right)\left(\frac{1}{\alpha}\right) pq=αβ+1+1+1αβpq = \alpha\beta + 1 + 1 + \frac{1}{\alpha\beta} pq=αβ+2+1αβpq = \alpha\beta + 2 + \frac{1}{\alpha\beta} Substitute the values of αβ\alpha\beta: pq=(3)+2+1(3)pq = (-3) + 2 + \frac{1}{(-3)} pq=113=43pq = -1 - \frac{1}{3} = -\frac{4}{3}

9. Form the New Quadratic Equation

The new quadratic equation is x2(p+q)x+pq=0x^2 - (p+q)x + pq = 0. Substitute the calculated sum (p+q=103p+q = \frac{10}{3}) and product (pq=43pq = -\frac{4}{3}): x2(103)x+(43)=0x^2 - \left(\frac{10}{3}\right)x + \left(-\frac{4}{3}\right) = 0 To eliminate the fractions, multiply the entire equation by 3: 3(x2103x43)=3×03 \left(x^2 - \frac{10}{3}x - \frac{4}{3}\right) = 3 \times 0 3x210x4=03x^2 - 10x - 4 = 0

10. Transform the Equation to Match the Correct Answer

Since the stated "Correct Answer" is (A) 3x220x12=03x^2 - 20x - 12 = 0, we need to work backwards and check for an error in the original question or answer key. Let's ASSUME that the correct new roots are p=3α+1βp = 3\alpha + \frac{1}{\beta} and q=3β+1αq = 3\beta + \frac{1}{\alpha}.

Then p+q=3(α+β)+α+βαβ=3(5)+53=1553=403p+q = 3(\alpha+\beta) + \frac{\alpha+\beta}{\alpha\beta} = 3(5) + \frac{5}{-3} = 15 - \frac{5}{3} = \frac{40}{3}. And pq=(3α+1β)(3β+1α)=9αβ+3+3+1αβ=9(3)+6+13=27+613=2113=643pq = (3\alpha + \frac{1}{\beta})(3\beta + \frac{1}{\alpha}) = 9\alpha\beta + 3 + 3 + \frac{1}{\alpha\beta} = 9(-3) + 6 + \frac{1}{-3} = -27 + 6 - \frac{1}{3} = -21 - \frac{1}{3} = -\frac{64}{3}.

The quadratic then becomes x2403x643=0x^2 - \frac{40}{3}x - \frac{64}{3} = 0, or 3x240x64=03x^2 - 40x - 64 = 0. This doesn't match (A).

Let's try the roots p=α+13βp = \alpha + \frac{1}{3\beta} and q=β+13αq = \beta + \frac{1}{3\alpha}. p+q=(α+β)+13α+βαβ=5+1353=559=409p+q = (\alpha+\beta) + \frac{1}{3}\frac{\alpha+\beta}{\alpha\beta} = 5 + \frac{1}{3} \frac{5}{-3} = 5 - \frac{5}{9} = \frac{40}{9}. pq=(α+13β)(β+13α)=αβ+13+13+19αβ=3+23+19(3)=3+23127=81+18127=6427pq = (\alpha+\frac{1}{3\beta})(\beta+\frac{1}{3\alpha}) = \alpha\beta + \frac{1}{3} + \frac{1}{3} + \frac{1}{9\alpha\beta} = -3 + \frac{2}{3} + \frac{1}{9(-3)} = -3 + \frac{2}{3} - \frac{1}{27} = \frac{-81+18-1}{27} = \frac{-64}{27}. The quadratic is x2409x6427=0x^2 - \frac{40}{9}x - \frac{64}{27} = 0, or 27x2120x64=027x^2 - 120x - 64 = 0. This doesn't match (A).

Let's assume there was a typo in the original equation, and the correct equation is actually x2(5+0)x43=0x^2 - (5 + 0)x - \frac{4}{3} = 0, or 3x215x4=03x^2 - 15x - 4 = 0, then α+β=5\alpha + \beta = 5 and αβ=43\alpha\beta = -\frac{4}{3}. In this case, we want the roots α+1β\alpha + \frac{1}{\beta} and β+1α\beta + \frac{1}{\alpha}. The sum is 5+54/3=5154=545 + \frac{5}{-4/3} = 5 - \frac{15}{4} = \frac{5}{4}. The product is 43+2+34=16+24912=112-\frac{4}{3} + 2 + \frac{-3}{4} = \frac{-16 + 24 - 9}{12} = \frac{-1}{12}. So the quadratic is x254x112=0x^2 - \frac{5}{4}x - \frac{1}{12} = 0, or 12x215x1=012x^2 - 15x - 1 = 0.

Let's assume the roots are α+1β\alpha+\frac{1}{\beta} and β+1α\beta+\frac{1}{\alpha}, and the quadratic is 3x220x12=03x^2 - 20x - 12 = 0. Then the sum of roots is 203\frac{20}{3} and the product is 4-4. Then α+β+α+βαβ=203\alpha+\beta + \frac{\alpha+\beta}{\alpha\beta} = \frac{20}{3} and αβ+2+1αβ=4\alpha\beta + 2 + \frac{1}{\alpha\beta} = -4. So αβ=3\alpha\beta = -3 is close, but doesn't work.

Let's try the roots 3α3\alpha and 3β3\beta. Then α+β=5\alpha+\beta = 5 and αβ=3\alpha\beta = -3, so 3α+3β=153\alpha + 3\beta = 15 and 9αβ=279\alpha\beta = -27. So this is wrong.

Let's assume the constant term is 16/316/3, not -12/3. Then 3x220x+16=03x^2 - 20x + 16 = 0. Then the roots are 3α3\alpha and β/3\beta/3 is not correct.

We are told that the correct answer is (A) 3x220x12=03x^2 - 20x - 12 = 0. This means p+q=20/3p+q = 20/3 and pq=4pq = -4. If α+β=5\alpha + \beta = 5 and αβ=3\alpha \beta = -3, then p+q=α+β+α+βαβ=5+53=103p+q = \alpha+\beta + \frac{\alpha+\beta}{\alpha \beta} = 5 + \frac{5}{-3} = \frac{10}{3}. pq=αβ+2+1αβ=3+213=43pq = \alpha \beta + 2 + \frac{1}{\alpha \beta} = -3 + 2 - \frac{1}{3} = -\frac{4}{3}. These are NOT the roots.

Instead of assuming α+1β\alpha+\frac{1}{\beta} and β+1α\beta+\frac{1}{\alpha}, let's assume the roots are αβ\frac{\alpha}{\beta} and βα\frac{\beta}{\alpha}. Then αβ+βα=α2+β2αβ=(α+β)22αβαβ=25+63=313\frac{\alpha}{\beta}+\frac{\beta}{\alpha} = \frac{\alpha^2+\beta^2}{\alpha\beta} = \frac{(\alpha+\beta)^2 - 2\alpha \beta}{\alpha \beta} = \frac{25 + 6}{-3} = -\frac{31}{3}. And αββα=1\frac{\alpha}{\beta} \cdot \frac{\beta}{\alpha} = 1. So the equation is x2+313x+1=0x^2 + \frac{31}{3} x + 1 = 0, or 3x2+31x+3=03x^2 + 31x + 3 = 0.

It seems there is an error in the problem statement or the correct answer. The correct derivation leads to 3x210x4=03x^2 - 10x - 4 = 0.

Common Mistakes & Tips

  • Be careful with signs when applying Vieta's formulas.
  • Double-check the simplification of logarithmic and exponential expressions. A small error there can propagate through the entire solution.
  • When transforming roots, systematically calculate the sum and product of the new roots.

Summary

The problem involves simplifying a quadratic equation with complex coefficients, finding the sum and product of its roots, and then finding a new quadratic equation whose roots are transformations of the original roots. The simplification relies on properties of logarithms and exponents. Based on the given information, the simplified quadratic equation is x25x3=0x^2 - 5x - 3 = 0, and the transformed quadratic equation is 3x210x4=03x^2 - 10x - 4 = 0. However, this contradicts the provided "Correct Answer: A". There appears to be an error either in the original problem statement or the answer key, as a rigorous derivation leads to the equation 3x210x4=03x^2 - 10x - 4 = 0.

Final Answer

The final answer is \boxed{3x^2 - 10x - 4 = 0}, which corresponds to option (B). However, the stated correct answer is (A).

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