NCERT Class XI Mathematics - Limits and Derivatives - Solutions

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Question : 37
Total: 72
For the function
f (x) =
x100
100
+
x99
99
+ ... +
x2
2
+ x + 1.
Prove that f ′(1) = 100 f ′(0).
Solution:  
We have
f (x) =
x100
100
+
x99
99
+ ... +
x2
2
+ x + 1. ... (i)
Differentiating (i) with respect to x we get
f' (x) =
100x99
100
+
99x98
99
+ ... +
2x
2
+ 1
⇒ f' (x) = 199+198 + ... + x + 1
At x = 1
f' (1) = 199+198 + ... + 1 + 1 = 100
and f' (0) = 0 + 0 + ... + 0 + 1 = 1
Hence f ′(1) = 100 f ′(0).
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