(1-1/2)x(1+1/2)x(1-1/3)x(1+1/3)x.x(1-1/99）x(1+1/99)
=1/2x3/2x2/3x3/4x4/3x.x98/99x100/99
=1/2x1x1x.x1x100/99
=50/99