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Question -

Find the derivatives of the following functions at the indicated points:
(i) sin x at x = π/2
(ii) x at x = 1
(iii) 2 cos x at x = π/2
(iv) sin 2xat x = π/2



Answer -

(i) sin x at x = π/2
Given:
f (x) = sin x
By using the derivative formula,

[Since it is ofindeterminate form. Let us try to evaluate the limit.]

We know that 1 – cos x = 2 sin2(x/2)

(ii) x at x = 1

Given:

f (x) = x

By using the derivative formula,

(iii) 2 cos x at x = π/2
Given:
f (x) = 2 cos x
By using the derivative formula,
(iv) sin 2xat x = π/2
Solution:
Given:
f (x) = sin 2x
By using the derivative formula,
[Since it is of indeterminate form. We shall apply sandwich theorem to evaluate the limit.]
Now, multiply numerator and denominator by 2, we get

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