(1-1/2的2次方)(1-1/3的2次方)......(1-n的2次方)=(1-1/2)(X+1/2)(1-1/3)(1+1/3)...(1-1/N)(1+1/n)=1/2x3/2x2/3x3/4x...x(n-1)/n (n+1)/n=1/2x(n+1)/n=(n+1)/2n
看成平方差公式做，例1-1/2的平方=(1-1/2)(1+1/2)以此类推
