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JEE Main 2023
Differentiation
Differentiation
Medium

Question

Let y(x)=(1+x)(1+x2)(1+x4)(1+x8)(1+x16)y(x) = (1 + x)(1 + {x^2})(1 + {x^4})(1 + {x^8})(1 + {x^{16}}). Then yyy' - y'' at x=1x = - 1 is equal to

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Solution

y=1x321x=1+x+x2+x3++x31y=1+2x+3x2++31x30y(1)=12+34++31=16y(x)=2+6x+12x2++31.30x29y(1)=26+1231.30=480y(1)y(1)=496\begin{aligned} & y=\frac{1-x^{32}}{1-x}=1+x+x^2+x^3+\ldots+x^{31} \\\\ & y^{\prime}=1+2 x+3 x^2+\ldots+31 x^{30} \\\\ & y^{\prime}(-1)=1-2+3-4+\ldots+31=16 \\\\ & y^{\prime \prime}(x)=2+6 x+12 x^2+\ldots+31.30 x^{29} \\\\ & y^{\prime \prime}(-1)=2-6+12 \ldots 31.30=480 \\\\ & y^{\prime \prime}(-1)-y^{\prime}(-1)=-496 \end{aligned}

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