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JEE Main 202225 JulyEvening Shift
Exam
JEE Main
Year
2022
Date
25 July
Shift
Evening Shift
Subject
Mathematics
Chapter
Quadratic Equation

JEE Main 2022 · 25 July · Evening Shift · Mathematics · Quadratic Equation

Let f(x) be a quadratic polynomial with leading coefficient 1 such...

Let f(x)beaquadraticpolynomialwithleadingcoefficientf\left(x\right) be a quadratic polynomial with leading coefficient1suchthatsuch thatf\left(0\right) = p, p \ne 0,and, andf\left(1\right) = \dfrac{1}{3}.Iftheequations. If the equationsf\left(x\right) = 0 and fofofof(x)=0fofofof\left(x\right) = 0 have a common real root, then f(−3)f\left(- 3\right) is equal to ______.

Correct answer

0

Step-by-step solution

Let f(x)=(x−α)(x−β)f\left(x\right) = \left(x - \alpha\right)\left(x - \beta\right)

It is given that f(0)=p⇒αβ=pf\left(0\right) = p ⇒ \alpha\beta = p

and f(1)=13⇒(1−α)(1−β)=13f\left(1\right) = \dfrac{1}{3} ⇒ \left(1 - \alpha\right)\left(1 - \beta\right) = \dfrac{1}{3}

Now let us assume that α\alpha is the common root of f(x)=0f\left(x\right) = 0 and fofofof(x)=0fofofof\left(x\right) = 0

fofofof(x)=0fofofof\left(x\right) = 0

⇒fofofof(α)=0⇒ fofofof\left(\alpha\right) = 0

⇒fofof(0)=0⇒ fofof\left(0\right) = 0

⇒fof(p)=0⇒ fof\left(p\right) = 0

So, f(p)f\left(p\right) is either α\alpha or β\beta.

Now assuming \left(p - \alpha\right)\left(p - \beta\right) = \alpha</p><p></p><p>⇒ \left(\alpha\beta - \alpha\right)\left(\alpha\beta - \beta\right) = \alpha ⇒ \left(\beta - 1\right)\left(\alpha - 1\right)\beta = 1</p><p></p><p>⇒ \dfrac{\beta}{3} = 1(as(1−α)(1−β)=13)\left(\text{as}\left(1 - \alpha\right)\left(1 - \beta\right) = \dfrac{1}{3}\right)

So, β=3\beta = 3

Now finding α\alpha by putting the value of βin(1−α)(1−β)=13\beta\text{in}\left(1 - \alpha\right)\left(1 - \beta\right) = \dfrac{1}{3},

⇒(1−α)(1−3)=13⇒ \left(1 - \alpha\right)\left(1 - 3\right) = \dfrac{1}{3}

⇒α=76⇒ \alpha = \dfrac{7}{6}

So, f(x)=(x−76)(x−3)f\left(x\right) = \left(x - \dfrac{7}{6}\right)\left(x - 3\right)

So, f(−3)=(−3−76)(−3−3)=25f\left(- 3\right) = \left(- 3 - \dfrac{7}{6}\right)\left(- 3 - 3\right) = 25

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