Ta có: \(M=\frac{1}{18}+\frac{1}{54}+\frac{1}{108}+\cdots+\frac{1}{990}\)

\(=\frac{1}{3\times6}+\frac{1}{6\times9}+\frac{1}{9\times12}+\cdots+\frac{1}{30\times33}\)

\(=\frac13\times\left(\frac{3}{3\times6}+\frac{3}{6\times9}+\cdots+\frac{3}{30\times33}\right)\)

\(=\frac13\times\left(\frac13-\frac16+\frac16-\frac19+\cdots+\frac{1}{30}-\frac{1}{33}\right)\)

\(=\frac13\times\left(\frac13-\frac{1}{33}\right)=\frac13\times\frac{10}{33}=\frac{10}{99}\)

3H = 3 (1/3-1/8-1/54-1/108-1/180-1/270-1/378) 
3H+(3/8) = 3/3 - 3/(6x9) - 3/(9x12) - 3/(12x15) - 3/(15x18) - 3/(1/18x21) 
3H+(3/8) = 1 - [(1/6)-(1/9)] - [(1/9)-(1/12)] - [(1/12)-(1/15)] - [(1/15)-(1/18)] - [(1/18)-(1/21)] 
= 1 -1/6 +1/9 -1/9 +1/12 -1/12 +1/15 -1/15 +1/18 -1/18 +1/21 
3H = -(3/8) + 1 -(1/6) +(1/21) 
3H = (-63 +168 -28 +8) / 168 
3H = 85/168 
đáp số: 
H = 85/504 
gần bằng 0,17

 i donn't

3H = 3 (1/3-1/8-1/54-1/108-1/180-1/270-1/378) 
3H+(3/8) = 3/3 - 3/(6x9) - 3/(9x12) - 3/(12x15) - 3/(15x18) - 3/(1/18x21) 
3H+(3/8) = 1 - [(1/6)-(1/9)] - [(1/9)-(1/12)] - [(1/12)-(1/15)] - [(1/15)-(1/18)] - [(1/18)-(1/21)] 
= 1 -1/6 +1/9 -1/9 +1/12 -1/12 +1/15 -1/15 +1/18 -1/18 +1/21 
3H = -(3/8) + 1 -(1/6) +(1/21) 
3H = (-63 +168 -28 +8) / 168 
3H = 85/168 
đáp số: 
H = 85/504 
gần bằng 0,17

B= 85/504

toán lớp 4 thôi