Home Maths Differentiation and Applications of Derivatives General Let f and g be two differentiable functions …
Maths Differentiation and Applications of Derivatives General Single Correct MCQ
Published on: August 14, 2026

Let f and g be two differentiable functions defined on an interval Ι such that f(x) ≥ 0 and g(x) ≤ 0 for all x ∈ Ι and f is strictly decreasing on Ι while g is strictly increasing on Ι then

A
the product function fg is strictly increasing on Ι
B
the product function fg is strictly decreasing on I
C
fog(x) is monotonically increasing on Ι
D
fog (x) is monotonically decreasing on Ι

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The correct answer is:
CHECK THE SOLUTION.

(a,d)Let h(x) = f(x) g(x)

h ′ (x) = f ′ (x) g(x) + g ′ (x) f(x)

As f ′ (x) < 0, g(x) ≤ 0 ⇒ f ′ (x) g(x) ≥ 0

and g ′ (x) > 0, f(x) ≥ 0 ⇒ f(x) g ′ (x) ≥ 0

⇒ h ′ (x) ≥ 0

⇒ h(x) is increasing.

Let x 1 , x 2 ∈ Ι

x 1 < x 2

g(x 1 ) < g(x 2 )

f(g(x 1 )) > f(g(x 2 ))

fog(x 1 ) > fog(x 2 )

⇒ fog(x) is monotonically decreasing.

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