(1-1/4)x(1-1/9)x(1-1/16)x.x(1-1/81)
=(1-1/2)x(1+1/2)x(1-1/3)x(1+1/3)x(1-1/4)x(1+1/4)x.x(1-1/9)x(1+1/9)
=1/2x3/2x2/3x4/3x3/4x5/4x.x8/9x10/9
=1/2x10/9
=5/9