Ta có:
\(A=\frac{6}{15.18}+\frac{6}{18.21}+\frac{6}{21.24}+...+\frac{6}{87.90}\)
\(A=2.\left(\frac{3}{15.18}+\frac{3}{18.21}+\frac{3}{21.24}+...+\frac{3}{87.90}\right)\)
\(A=2.\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+...+\frac{1}{87}-\frac{1}{90}\right)\)
\(A=2.\left(\frac{1}{15}-\frac{1}{90}\right)\)
\(A=2.\frac{1}{18}=\frac{1}{9}\)
6/15.18 + 6/18.21 + 6/21.24 + ...+ 6/87.90
tính 6/15.18+6/18.21+6/21.24+...+6/87.90
Tính tổng : A=6/15.18 + 6/18.21 + 6/21.24 +...+6/87.90
B=2/15+2/35+2/63+....+2/399
c=7/10.11+7/11.12+7/12.13 + .....+7/69.70
D=6/15.18 + 6/ 18.21+6/21.24 +...+6/87.90
Tính nhanh:
B= 6/1.3 + 6/3.5 + 6/5.7 + 6/7.9 + .....+ 6/99.101
C= 6/15.18 + 6/ 18.21 + 6/21.24+ ......+ 6/87.90
1/15.18+1/18.21+1/21.24+..+1/87.90
tính nhanh
Tính tổng:
a) A = \(\frac{1}{2}\)+ \(\frac{1}{2.3}\)+ \(\frac{1}{3.4}\)+ .... + \(\frac{1}{23.24}\)
b) B = \(\frac{6}{15.18}\)+ \(\frac{6}{18.21}\)+ \(\frac{6}{21.24}\)+ .... + \(\frac{6}{87.90}\)
A=7/10.11+7/11.12+7/12.13+...+7/69.70. B=6/15.18+6/18.21+...+6/87.90
C=15/90.94+15/94.98+...+15/146.150. D=10/56+10/140+10/260+...+10/140
Tính hợp lí
\(A=\left(\frac{21}{31}+\frac{-16}{7}\right)+\left(\frac{44}{53}+\frac{10}{21}\right)+\frac{9}{53}\)
\(B=\frac{6}{15.18}+\frac{6}{18.21}+...+\frac{6}{87.90}\)
