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JEE Main 2024
Differentiation
Differentiation
Hard

Question

Let f:RRf: \mathbb{R} \rightarrow \mathbb{R} be a thrice differentiable function such that f(0)=0,f(1)=1,f(2)=1,f(3)=2f(0)=0, f(1)=1, f(2)=-1, f(3)=2 and f(4)=2f(4)=-2. Then, the minimum number of zeros of (3ff+ff)(x)\left(3 f^{\prime} f^{\prime \prime}+f f^{\prime \prime \prime}\right)(x) is __________.

Answer: 0

Solution

f:RR and f(0)=0,f(1)=1,f(2)=1\because f: R \rightarrow R \text { and } f(0)=0, f(1)=1, f(2)=-1 \text {, } f(3)=2f(3)=2 and f(4)=2f(4)=-2 then f(x)f(x) has atleast 4 real roots. Then f(x)f(x) has atleast 3 real roots and f(x)f^{\prime}(x) has atleast 2 real roots. Now we know that ddx(f3f)=3f2ff+f3f=f2(3ff+ff)\begin{aligned} \frac{d}{d x}\left(f^3 \cdot f^{\prime \prime}\right) & =3 f^2 \cdot f^{\prime} \cdot f^{\prime \prime}+f^3 \cdot f^{\prime \prime \prime} \\ & =f^2\left(3 f^{\prime} \cdot f^{\prime}+f \cdot f^{\prime \prime}\right) \end{aligned} Here f3ff^3 \cdot f' has atleast 6 roots. Then its differentiation has atleast 5 distinct roots.

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