Question

观察:六分之一等于二乘三分之一等于二分之一减三分之一

Answer 1

1/2+1/6+1/12……+1/90

=1/(1*2)+1/(2*3)+1/(3*4)+……+1/(9*10)

=1-1/2+1/2-1/3+1/3-1/4+……+1/9-1/10

=1-1/10

=9/10