解
x=√y
即y=x² (x>=0)
令y=2解得 x=√2
所以
S=∫(√2,0)(2-x²)dx
=(-1/3)(√2)³+2√2
=4√2/3
V=∫(2,0)πx²dy
=π∫(2,0)ydy
=π(1/2)*2²
=2π