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NCERT Class XI Mathematics - Limits and Derivatives - Solutions

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Question : 37 of 72
Marks: +1, -0
For the function
f (x) = x100100+x9999\frac{x^{100}}{100}+\frac{x^{99}}{99} + ... + x22\frac{x^{2}}{2} + x + 1.
Prove that f ′(1) = 100 f ′(0).
Solution:  
We have
f (x) = x100100+x9999\frac{x^{100}}{100}+\frac{x^{99}}{99} + ... + x22\frac{x^{2}}{2} + x + 1. ... (i)
Differentiating (i) with respect to x we get
f' (x) = 100x99100+99x9899\frac{100x^{99}}{100}+\frac{99x^{98}}{99} + ... + 2x2\frac{2x}{2} + 1
⇒ f' (x) = 199+1981^{99}+1^{98} + ... + x + 1
At x = 1
f' (1) = 199+1981^{99}+1^{98} + ... + 1 + 1 = 100
and f' (0) = 0 + 0 + ... + 0 + 1 = 1
Hence f ′(1) = 100 f ′(0).
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