Question

[Hiếu Nguyễn]

tính nhanh tổng sau : a= 1/3 + 1/8 +1/15 + 1/24 + 1/35 + 1/48 + 1/63 + 1/80

Answer 1

[Lê Minh Phúc]

2a= 2/3+2/8+2/15+2/24+2/35+2/48+2/63+2/80= [2/( 1*3)+2/( 3*5)+2/( 5*7)+2/( 7*9)]+[2/(2*4)+2/(4*6)+2/(6*8)+2/(8*10)]= [1/1-1/3+1/3-1/5+1/5-1/7+1/7-1/9]+[1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10]= [1/1-1/9]+[1/2-1/10]= 8/9+2/5= 58/45 =>a= 29/45