Writing Errands

Hi tutor, can you please help me to solve that question and show all the work?
Let f (x) be a function on (0, 100), having derivative f ′ (x) and a primitive function F (x) = x f (t)dt 0
defined on the same domain. For all x ∈ (0,100), it is known that f(x) ≥ 0 and f′(x) ≤ 0.
(a)we learned that f′(x) ≤ 0 for all x ∈ (0,100) implies that f(x) is decreasing. Prove this statement by using the properties of definite integrals. In other words, for all a,b ∈ (0,100), prove that f(a) ≥ f(b) if a < b.

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