Question

A=1/21+1/28+1/36+...+2/x(x+1)=2/9

Answer 1

x = 17 

còn giải thì dài dòng lắm 

Answer 2

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

2/42 + 2/56 + 2/72 + ... + 2/x(x + 1) = 2/9

2 × [1/6×7 + 1/7×8 + 1/8×9 + ... + 1/x(x + 1)] = 2/9

1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + ... + 1/x - 1/x + 1 = 2/9 : 2

1/6 - 1/x + 1 = 2/9 × 1/2 = 1/9

1/x + 1 = 1/6 - 1/9

1/x + 1 = 1/18

=> x + 1 = 18

=> x = 18 - 1 = 17

Answer 3

=>2A=2/42+2/56+2/72+...+4/x(x+1)=4/9

=>2A=2/6.7+2/7.8+2/8.9+...+4/x(x+1)=4/9

=>2A=2[1/6-1/7+1/7-1/8+1/8-1/9+...+2/x(x+1)] =4/9

=>2A=2[1/6+2/x(x+1)]=4/9

=>A=1/6+2/x(x+1)=2/9

=>A=2/x(x+1)=2/9-1/6=1/18

=>x(x+1)=18.2=36=>x=???