Question

4/1*3+4/3*5+4/5*7+.+4/19*21等于多少?

Answer 1

=4(1/1*3+1/3*5+……+1/19*21）=4[（1-1/3)/2+(1/3-1/5)/2+……+(1/19-1/21)/2]=4/2*(1-1/3+1/3-1/5+……+1/19-1/21）=2*（1-1/21）=40/21