Question

tinh tong A=1/2+1/6+1/12+1/20+...+1/72+1/90

Answer 1

A=1/2+1/6+1/12+1/20+...+1/72+1/90

A=1/1x2+1/2x3+1/3x4+1/4x5+.....+1/8x9+1/9x10

A=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+....+1/8-1/9+1/9-1/10

A=1-1/10

A=9/10

k cho mik nha

Answer 2

A=1/(1*2)+1/(2*3)+1/(3*4)+.....+1/(9*10)

=1/1-1/2+1/2-1/3+.......+1/9-1/10

=1/1-1/10=9/10