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QJEEM22Mac4a9: JEE Main 2022 Mathematics - Class 11 JEEM22Mac4a9 Solution: Let be a quadratic polynomial with leading coefficient 1 such…

Let f(x)f(x) be a quadratic polynomial with leading coefficient 1 such that f(0)=p,p≠0f(0)=p, p \neq 0 , and f(1)=13f(1)=\frac{1}{3} . If the equations f(x)=0f(x)=0 and f∘f∘f∘f(x)=0f \circ f \circ f \circ f(x)=0 have a common real root, then f(−3)f(-3) is equal to ________________.

Step-by-Step Verified Solution

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

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

and f(1)=13⇒(1−α)(1−β)=13f(1) = {1 \over 3} \Rightarrow (1 - \alpha )(1 - \beta ) = {1 \over 3}

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

fofofof(x)=0fofofof(x) = 0

⇒fofof(0)=0\Rightarrow fofof(0) = 0

⇒fof(p)=0\Rightarrow fof(p) = 0

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

(p−α)(p−β)=α(p - \alpha )(p - \beta ) = \alpha

(αβ−α)(αβ−β)=α⇒(β−1)(α−1)β=1(\alpha \beta - \alpha )(\alpha \beta - \beta ) = \alpha \Rightarrow (\beta - 1)(\alpha - 1)\beta = 1 ( ∵\because α≠0\alpha \ne 0 )

So, β=3\beta = 3

(1−α)(1−3)=13(1 - \alpha )(1 - 3) = {1 \over 3}

α=76\alpha = {7 \over 6}

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

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

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