I think that the inequality is incorrect
See the graph here : https://www.desmos.com/calculator/alijngoclf
Notice that x^2 is ≥ sin^2x everywhere on (-pi/2, pi/2)
But if x^2 ≤ f(x) on this interval...then f(x) is ≥ x^2 everywhere on this interval...then f(x) cannot also be ≤ sin^2x everywhere on the interval
I believe that the inequality we need is this
sin^2 x ≤ f(x) ≤ x^2
Note that if this true......then
f(x) f(x) = 0 by the Squeeze Theorem
lim x → 0