D={(x,y):0＜x＜1,-\(\sqrt{\quad}\)(x-\(x^{2}\))＜y＜\(\sqrt{\quad}\)(x-\(x^{2}\))}
∬\(\sqrt{\quad}\)xdxdy
=∫dx\(\sqrt{\quad}\)x∫dy
=∫(\(\sqrt{\quad}\)x)2\(\sqrt{\quad}\)(x-\(x^{2}\))dx
=∫2x\(\sqrt{\quad}\)(1-\(x^{2}\))dx
=[(-\(\frac{2}{3}\))(1-\(x^{2}\))^(\(\frac{3}{2}\))]
=0-(-\(\frac{2}{3}\))
=\(\frac{2}{3}\)