Question

（1-2平的方分之1）乘（1-3的平方分之1）乘（1减4的平方分之1）乘.乘（1减19

Answer 1

(1-1/2^2)(1-1/3^2)(1-1/4^2)*.*(1-1/19^2)

=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)*.*(1-1/19)(1+1/19)

=1/2*3/2*2/3*4/3*3/4*5/4*.*18/19*20/19

=1/2*20/19

=10/19