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Application Of Derivatives

Question
CBSEENMA12035281

On which of the following intervals is the function x100 + sin x – 1 strictly increasing?
(i)  (1,  1)     (ii) (0, 1)        (iii) open parentheses straight pi over 2 comma space straight pi close parentheses     (iv) open parentheses 0 comma space space straight pi over 2 close parentheses

Solution

Let f (x) = x100 + sin x – 1 ∴ f ' (x) = 100 x99 + cos x
(i) For – 1 < x < 1, f (x) > 0 is not necessarily true
∴  f (x) is not strictly increasing on (– 1, 1)
(ii) For 0 < x < 1, f ' (x) > 0 ∴ f (x) is strictly increasing on (0, 1).
(iii) 1 space or space space straight pi over 2 space less than space straight x thin space less than space straight pi comma space space space straight f apostrophe left parenthesis straight x right parenthesis space greater than space 0 space space space space space space space therefore space space space space straight f left parenthesis straight x right parenthesis space is space strictly space increasing space on space open parentheses straight pi over 2 comma space straight pi close parentheses
(iv) For space 0 space less than space straight x less than space straight pi over 2 comma space space space straight f apostrophe left parenthesis straight x right parenthesis thin space greater than 0 space space space space space space space therefore space space space space straight f left parenthesis straight x right parenthesis space is space strictly space increasing space on space open parentheses 0 comma space space straight pi over 2 close parentheses.

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