x=cosa
√(1-x²)=sina
dx=-sinada
x=1,a=0
x=0,a=π/2
原式=∫(π/2→0)(-sin²acos²a)da
=1/4∫(0→π/2)sin²2ada
=1/8∫(0→π/2)(1-cos4a)da
=1/32∫(0→π/2)(1-cos4a)d4a
=1/32(4a-sin4a)(0→π/2)
=1/32*(2π-0)-1/32*(0-0)
=π/16