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Differentiation question

2023 · 25 Jan · Shift 1 · Q32
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  5. /2023 · 25 Jan · Shift 1 · Q32

Differentiation question

2023 · 25 Jan · Shift 1 · Q32

JEE MainMathematicsDifferentiationMCQ+4 / −1
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}})y(x)=(1+x)(1+x2)(1+x4)(1+x8)(1+x16). Then y′−y′′y' - y''y′−y′′ at x=−1x = - 1x=−1 is equal to
  1. A
    496
  2. B
    976
  3. C
    464
  4. D
    944
View written solutionFree

Correct answer: A

  1. Simplify the product

We use the identity

(1−x)(1+x)(1+x2)(1+x4)(1+x8)(1+x16)=1−x32.(1-x)(1+x)(1+x^2)(1+x^4)(1+x^8)(1+x^{16})=1-x^{32}.(1−x)(1+x)(1+x2)(1+x4)(1+x8)(1+x16)=1−x32.

Hence,

(1+x)(1+x2)(1+x4)(1+x8)(1+x16)=1−x321−x,x≠1.(1+x)(1+x^2)(1+x^4)(1+x^8)(1+x^{16})= \frac{1-x^{32}}{1-x}, \quad x\neq 1.(1+x)(1+x2)(1+x4)(1+x8)(1+x16)=1−x1−x32​,x=1.

So

y(x)=1−x321−x=1+x+x2+⋯+x31.y(x)=\frac{1-x^{32}}{1-x}=1+x+x^2+\cdots+x^{31}.y(x)=1−x1−x32​=1+x+x2+⋯+x31.

This polynomial form is valid for all xxx, so we can differentiate termwise.

  1. Differentiate

Since

y(x)=∑k=031xk,y(x)=\sum_{k=0}^{31} x^k,y(x)=k=0∑31​xk,

we get

y′(x)=∑k=131kxk−1,y'(x)=\sum_{k=1}^{31} kx^{k-1},y′(x)=k=1∑31​kxk−1,

and

y′′(x)=∑k=231k(k−1)xk−2.y''(x)=\sum_{k=2}^{31} k(k-1)x^{k-2}.y′′(x)=k=2∑31​k(k−1)xk−2.

We need y′(−1)−y′′(−1)y'(-1)-y''(-1)y′(−1)−y′′(−1).

  1. Compute y′(−1)y'(-1)y′(−1)
y′(−1)=∑k=131k(−1)k−1.y'(-1)=\sum_{k=1}^{31} k(-1)^{k-1}.y′(−1)=k=1∑31​k(−1)k−1.

This is the alternating sum

1−2+3−4+⋯+31.1-2+3-4+\cdots+31.1−2+3−4+⋯+31.

Pair terms:

(1−2)+(3−4)+⋯+(29−30)+31.(1-2)+(3-4)+\cdots+(29-30)+31.(1−2)+(3−4)+⋯+(29−30)+31.

There are 151515 pairs, each equal to −1-1−1, so

y′(−1)=15(−1)+31=16.y'(-1)=15(-1)+31=16.y′(−1)=15(−1)+31=16.
  1. Compute y′′(−1)y''(-1)y′′(−1)
y′′(−1)=∑k=231k(k−1)(−1)k−2.y''(-1)=\sum_{k=2}^{31} k(k-1)(-1)^{k-2}.y′′(−1)=k=2∑31​k(k−1)(−1)k−2.

Since (−1)k−2=(−1)k(-1)^{k-2}=(-1)^k(−1)k−2=(−1)k, this becomes

y′′(−1)=∑k=231k(k−1)(−1)k.y''(-1)=\sum_{k=2}^{31} k(k-1)(-1)^k.y′′(−1)=k=2∑31​k(k−1)(−1)k.

Write terms in pairs:

(2⋅1)−(3⋅2)+(4⋅3)−(5⋅4)+⋯+(30⋅29)−(31⋅30).(2\cdot1)-(3\cdot2)+(4\cdot3)-(5\cdot4)+\cdots+(30\cdot29)-(31\cdot30).(2⋅1)−(3⋅2)+(4⋅3)−(5⋅4)+⋯+(30⋅29)−(31⋅30).

Now pair as

(2⋅1−3⋅2)+(4⋅3−5⋅4)+⋯+(30⋅29−31⋅30).(2\cdot1-3\cdot2)+(4\cdot3-5\cdot4)+\cdots+(30\cdot29-31\cdot30).(2⋅1−3⋅2)+(4⋅3−5⋅4)+⋯+(30⋅29−31⋅30).

For a general pair with even k=2mk=2mk=2m:

(2m)(2m−1)−(2m+1)(2m)=2m[(2m−1)−(2m+1)]=−4m.(2m)(2m-1)-(2m+1)(2m)=2m[(2m-1)-(2m+1)]=-4m.(2m)(2m−1)−(2m+1)(2m)=2m[(2m−1)−(2m+1)]=−4m.

Here m=1,2,…,15m=1,2,\dots,15m=1,2,…,15. Therefore

y′′(−1)=∑m=115(−4m)=−4⋅15⋅162=−480.y''(-1)=\sum_{m=1}^{15}(-4m)=-4\cdot\frac{15\cdot16}{2}=-480.y′′(−1)=m=1∑15​(−4m)=−4⋅215⋅16​=−480.
  1. Compute the required value
y′(−1)−y′′(−1)=16−(−480)=496.y'(-1)-y''(-1)=16-(-480)=496.y′(−1)−y′′(−1)=16−(−480)=496.
  1. Compare with the stored answer

Derived answer is 496, which matches Option A.

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