`a) A=1/(1.2)+1/(2.3)+1/(3.4)+.....+1/(99.100)`
`A=1-1/2+1/2-1/3+1/3-1/4+.......+1/99-1/100`
` A=1-1/100`
`A=100/100-1/100 `
` A=99/100`
b) `B=1+1/2(1+2)+1/3(1+2+3)+.......+1/20(1+2+3+4+....+20)`
`B=1+1/2((2.3)/2)+1/3((3.4)/2)+.......1/20((20.21)/2)`
` B=1+(1.2.3)/(2.2)+(1.3.4)/(3.2)+.......(1.20.21)/(20.2)`
`B=1+3/2+4/2+5/2+......21/2`
`B=1/2.(2+3+4+5+......+21) `
`B=1/2 .230`
` B=115`
Mong bạn vote 5* và ctlhn!
$#Minh Thắng#$
