If y = (x) is a nonnegative continuous function over the closed interval a x

If y = ƒ(x) is a nonnegative continuous function over the closed interval a ≤ x ≤ b, show that the double integral definition of area for the closed plane region bounded by the graph of ƒ, the vertical lines x = a and x = b, and the x-axis agrees with the definition for area beneath the curve.