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JEE Advanced 2024 · Continuity and Differentiability

Let R\mathbb{R} denote the set of all real numbers. Consider the polynomial function f:R→Rf:\mathbb{R}\to\mathbb{R} defined by f(x)=d10dx10((x2−1)10)f(x)=\frac{d^{10}}{dx^{10}}\left((x^{2}-1)^{10}\right), for all x∈Rx\in\mathbb{R}. Here d10dx10((x2−1)10)\frac{d^{10}}{dx^{10}}\left((x^{2}-1)^{10}\right) is the 10th10^{\text{th}} order derivative of the function (x2−1)10(x^{2}-1)^{10}. Then which of the following statements is (are) TRUE?

Answers: A. The coefficient of x8x^{8} in the polynomial f(x)f(x) is (−10)(18!8!)(-10)\left(\frac{18!}{8!}\right) · B. The value of f(1)+f(−1)f(1)+f(-1) is equal to 10! 21110!\,2^{11} · C. The degree of the polynomial f(x)f(x) is 10

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Written solution

f(x)=d10dx10((x2−1)10)f(x)=\dfrac{d^{10}}{dx^{10}}\left((x^{2}-1)^{10}\right)

g(x)=(x2−1)10g(x)=(x^{2}-1)^{10}

Check: coeff of x8, f(1)+f(−1), deg⁡f, constant term\text{Check: coeff of } x^{8},\ f(1)+f(-1),\ \deg f,\ \text{constant term}

g(x)=∑k=010(10k)(−1)10−kx2kg(x)=\sum_{k=0}^{10}\binom{10}{k}(-1)^{10-k}x^{2k}

[xm]f=am+10⋅(m+10)!m![x^{m}]f=a_{m+10}\cdot\dfrac{(m+10)!}{m!}

deg⁡g=20⇒deg⁡f=20−10=10\deg g=20 \Rightarrow \deg f=20-10=10

⇒(3) TRUE\Rightarrow (3) \text{ TRUE}

a18=(109)(−1)1=−10a_{18}=\binom{10}{9}(-1)^{1}=-10

[x8]f=a18⋅18!8!=(−10)18!8![x^{8}]f=a_{18}\cdot\dfrac{18!}{8!}=(-10)\dfrac{18!}{8!}

⇒(1) TRUE\Rightarrow (1) \text{ TRUE}

x=1+t: (x2−1)10=(2t+t2)10=t10(2+t)10x=1+t:\ (x^{2}-1)^{10}=(2t+t^{2})^{10}=t^{10}(2+t)^{10}

f(1)=10! 210,f(−1)=10! 210f(1)=10!\,2^{10},\quad f(-1)=10!\,2^{10}

⇒f(1)+f(−1)=10! 211⇒(2) TRUE\Rightarrow f(1)+f(-1)=10!\,2^{11} \Rightarrow (2) \text{ TRUE}

a10=(105)(−1)5=−252a_{10}=\binom{10}{5}(-1)^{5}=-252

f(0)=10!⋅(−252)=−(10!5!)2f(0)=10!\cdot(-252)=-\left(\dfrac{10!}{5!}\right)^{2}

≠−10!5!⇒(4) FALSE\neq -\dfrac{10!}{5!} \Rightarrow (4) \text{ FALSE}

TRUE: (1), (2), (3)\text{TRUE: } (1),\ (2),\ (3)

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