Question

(1/3x5+1/5x7+1/7x9+.....+1/19x21)*x=9/7

Answer 1

(1/3x5+1/5x7+....+1/19x21)*x=9/7

(1/3-1/5+1/5-1/7+...+1/19-1/21)*x=9/7

(1/3-1/21)*x=9/7

2/7*x=9/7

=> x=9/7:2/7

=> x=9/2

Answer 2

Bạn leminhduc sai rùi @@

Ta xét : 

B = \(\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{19.21}\)

2 x B = \(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21}\)

2 x B = \(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}\)

2 x B = \(\frac{1}{3}-\frac{1}{21}\)=\(\frac{2}{7}\)

B = \(\frac{2}{7}:2\)

B = \(\frac{1}{7}\)

Thay B vào biểu thức ta có :

\(\frac{1}{7}.x=\frac{9}{7}\)

=> x = \(\frac{9}{7}:\frac{1}{7}\)=\(\frac{9}{7}.\frac{7}{1}\)=9

Vậy x = 9

Answer 3

Ủng hộ mik nhá ^_^"