∫(0→a) x²√(a² - x²) dx,令x = asinz,dx = acosz dz= ∫(0→π/2) (a²sin²z)|acosz| * (acosz dz)= a⁴∫(0→π/2) sin²zcos²z dz= a⁴∫(0→π/2) (1/4)sin²...