(1-1/2^2)(1-1/3^2)……(1-1/n^2)
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)...(1-1/n)(1+1/n)
=1/2*3/2*2/3*4/3*...*(1-1/n)(1+1/n)
=1/2*(1+1/n)
当n趋向无穷大时
极限=1/2(1+0)=1/2