Question

tìm x biết :

4/3.7+4/7.11+4/11.15+...+4/(3x-1).(3x+3)=3/10

Answer 1

\(\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+...+\frac{4}{\left(3x-1\right).\left(3x+3\right)}=\frac{3}{10}\)

=> \(\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+...+\frac{1}{3x-1}-\frac{1}{3x+3}=\frac{3}{10}\)

=> \(\frac{1}{3}-\frac{1}{3x+3}=\frac{3}{10}\)

=> \(\frac{1}{3x+3}=\frac{1}{3}-\frac{3}{10}\)

=> \(\frac{1}{3x+3}=\frac{1}{30}\)

=> 3x + 3 = 30

=> 3.(x + 1) = 30

=> x + 1 = 10

=> x = 9

Answer 2

x = 9 nha bạn