Question

[Hoang Van Hieu]

Tim X

a,X - 20/11 x 13 - 20/13 x 15 - 20/15 x 17 -...-20/53 x 55 = 3/11

b,1/21 + 1/28 +1/36 + ... +2/Xx(x + 1) = 2/9

Answer 1

[Trung]

bn l-ike cho mk trước đã

Answer 2

[Phạm Việt Anh]

Ta có : A=20/11×13 + 20/13×15 +20/15×17+...+20/53×55

A = 10 ×( 2/11×13+2/13×15+...12/53×55)

A = 10 ×(1/11-1/13+1/13-1/15+1/15-1/17+...+1/53-1/55)

A = 10 × (1/11-1/55)

A =10 × 4/55

A = 8/11