tính nhanh 1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90

`1/(2) + 1/6 + 1/(12) + 1/(20) + 1/(30) + 1/(42) + 1/(56) + 1/(72) + 1/(90)`

`= 1/(1 xx 2) + 1/(2 xx 3) + 1/(3 xx  4) + 1/(4 xx 5) + 1/(5 xx 6) + 1/(6 xx7 ) + 1/(7 xx 8) + 1/(8 xx 9) + 1/(9xx 10)`

`= 1 - 1/2 + 1/2 - 1/3 + 1/3 -1/4 + 1/4 -1/5 + 1/5 - 1/6 + 1/6 -1/7 + 1/7 -1/8 +1/8 -1/9 + 1/9 - 1/(10)`

`= 1 - 1/(10)`

`= 9/(10)`