\subsection*Solution 7 This is the standard definition of the Riemann integral using right endpoints. Since (f) is continuous, it is Riemann integrable, and the limit of any sequence of Riemann sums with mesh (\to 0) equals the integral.

Show π/6 ≤ ∫₀^(π/2) sin x / (1+x²) dx ≤ π/2.

\subsection*Solution 9 Since (f \ge 0), any lower sum (L(P,f) \ge 0). The integral is the supremum of lower sums, hence (\int_a^b f = \sup L(P,f) \ge 0).