mình chỉ tính vế bên thôi nhé bn tích x
\(\frac{1}{4.7}+\frac{1}{7.10}+...+\frac{1}{25.28}\)
\(\frac{1}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-.....+\frac{1}{25}-\frac{1}{28}\right)\)
=\(\frac{1}{3}\left(\frac{1}{4}-\frac{1}{28}\right)\)
=\(???\)
ban oi minh cung lam giong zay nhung sai
\(\frac{x+1}{4\times7}+\frac{1}{7\times10}+\frac{1}{10\times13}+...+\frac{1}{25\times28}=\frac{15}{14}\)
\(\frac{x+1}{4\times7}+\left(\frac{1}{7\times10}+\frac{1}{10\times13}+...+\frac{1}{25\times28}\right)=\frac{15}{14}\)
\(\frac{x+1}{4\times7}+\left(\frac{3}{7\times10}+\frac{3}{10\times13}+...+\frac{3}{25\times28}\right)=\frac{15}{14}\)
\(\frac{x+1}{4\times7}+\frac{1}{28}=\frac{15}{14}\)
\(\frac{x+1}{4\times7}=\frac{15}{14}-\frac{1}{28}\)
\(\frac{x+1}{4\times7}=\frac{29}{28}\)
x = 29 - 1
x = 28
x+1/4.7+1/7.10+1/10.13+...+1/25.28=15/14
3.x+3/4.7+3/7.10+3/10.13+...+3/25.28=45/14
3x+3/4.7+1/7-1/10+1/10-1/10+...+1/25-1/28=45/14
3x+3/4.7+(1/7-1/28)=45/14
3(x+1)/28+3/28=45/14
3(x+1)+3/28=45/14
3(x+2)/28=45/14
3(x+2)=90
x+2=30
x=28