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Find the interval in which function f(x) = sinx+cosx, 0 ≤ x ≤ 2π is decreasing.(a) (π/4, 5π/4)(b) (-π/4, 5π/4)(c) (π/4, -5π/4)(d) (-π/4, π/4)I got this question during an interview.Question is taken from Derivatives Application in portion Application of Derivatives of Mathematics – Class 12

Answer»

Correct option is (a) (π/4, 5π/4)

To EXPLAIN: f(x) = sinx+COSX.

f’(x) = cosx – sinx. Now f’(x) = 0 gives sinx = cosx which gives that x=π/4, 5π/4 as 0 ≤ x ≤ 2π.

Therefore on CHECKING the values we get f is DECREASING in (π/4, 5π/4).



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