Question

C= 1/10+1/15+1/21+...+1/120

làm đầy đủ 

Answer 1

1/10=2/20=2/(4.5)

1/15=2/30=2/(5.6)

1/21=2/42=2/(6.7)

....

1/120=2/240=2/(15.16)

=> C=1/10+1/15+...+1/120=2[1/(4.5)+1/(5.6)+...+1/(15.16)

       =2[1/4-1/16)=2(3/16)=3/8

Answer 2

nhân cả 2 vế với 2 ta có 

2C=1/20+1/30+1/42+....+1/240

2C=1/4X5+1/5X6+1/6X7+....+1/15X16

2C=1/4-1/5+1/5-1/6+1/6-1/7+...+1/15-1/16

2C=1/4-1/16 (khử liên tiếp)

2C=3/16

C=3/16:2

C=3/32

Answer 3

Cx2= (1/10+1/15+1/21+...+1/120)x2=2/20+2/30+2/42+.....+2/240=2/4x5+2/5x6+...+2/15x16=2x(1/4x5+1/5x6+.....+1/15x16)=2x(1/4-1/5+1/5-1/6+....+1/15-1/16)=2x(1/4-1/16)=2x(3/16)=0,375

=>C=0,375

Vậy C=0,375

Answer 4

0,375 tick cho mình nhé