Ta có :
\(\frac{3}{18x21}+\frac{3}{21x24}+...+\frac{3}{123x126}\)
\(=\frac{3}{18}-\frac{3}{21}+\frac{3}{21}-\frac{3}{24}+...+\frac{3}{123}-\frac{3}{126}\)
\(=\frac{3}{18}-\frac{3}{126}\)
\(=\frac{1}{7}\)
~ Thiên Mã ~
3/(18x21) +3/(21x24) + 3/(24 x 27) + .... +3/(123x 126)
=1/18-1/21+1/21-1/24+...+1/123-1/126
=1/18-1/126
=7/126-1/126
=6/126=1/21
Ohhhh ! Sr bn nhoa =(( Mk lm sai r
Bn Tôi_Mệt_ Rồi lm ms đúng nhoa
~ Thiên Mã ~
\(\frac{3}{18.21}+\frac{3}{21.24}+...+\frac{3}{123.126}\)
\(=\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+...+\frac{1}{123}-\frac{1}{126}\)
\(=\frac{1}{18}-\frac{1}{126}\)
\(=\frac{1}{21}\)
\(\frac{3}{18\cdot21}+\frac{3}{21\cdot24}+\frac{3}{24\cdot27}+...+\frac{3}{123\cdot126}\)
\(=\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+...+\frac{1}{123}-\frac{1}{126}\)
\(=\frac{1}{18}-\frac{1}{126}\)
\(=\frac{7}{126}-\frac{1}{126}=\frac{6}{126}=\frac{2\cdot3}{2\cdot63}=\frac{1\cdot1}{1\cdot21}=\frac{1}{21}\)