Question

2分之1+6分之1+12分之1+20分之1+.+90分之1得多少?

Answer 1

原式=1/2+1/2×3+1/3×4+1/4×5+-------+1/9×10

=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+------1/9+1/9-1/10

=1-1/10

=9/10