Question

 ai giúp mình với : x+1/4*7+1/7*10+1/10*13.....+1/25*28=15/14

Answer 1

mình chỉ tính vế bên thôi nhé bn tích x

\(\frac{1}{4.7}+\frac{1}{7.10}+...+\frac{1}{25.28}\)

\(\frac{1}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-.....+\frac{1}{25}-\frac{1}{28}\right)\)

=\(\frac{1}{3}\left(\frac{1}{4}-\frac{1}{28}\right)\)

=\(???\)

Answer 2

ban oi sai roi

Answer 3

ban oi minh cung lam giong zay nhung sai

Answer 4

\(\frac{x+1}{4\times7}+\frac{1}{7\times10}+\frac{1}{10\times13}+...+\frac{1}{25\times28}=\frac{15}{14}\)

\(\frac{x+1}{4\times7}+\left(\frac{1}{7\times10}+\frac{1}{10\times13}+...+\frac{1}{25\times28}\right)=\frac{15}{14}\)

\(\frac{x+1}{4\times7}+\left(\frac{3}{7\times10}+\frac{3}{10\times13}+...+\frac{3}{25\times28}\right)=\frac{15}{14}\)

\(\frac{x+1}{4\times7}+\frac{1}{28}=\frac{15}{14}\)

\(\frac{x+1}{4\times7}=\frac{15}{14}-\frac{1}{28}\)

\(\frac{x+1}{4\times7}=\frac{29}{28}\)

x = 29 - 1

x = 28

Answer 5

lan huong oi sai roi

Answer 6

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