\(A=\frac{4}{2.5}+\frac{4}{5.8}+\frac{4}{8.11}+...+\frac{4}{65.68}\)
\(A=4\left(\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+...+\frac{1}{65.68}\right)\)
\(A=\frac{4}{3}\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{65}-\frac{1}{68}\right)\)
\(A=\frac{4}{3}\left(\frac{1}{2}-\frac{1}{68}\right)\)
\(A=\frac{4}{3}.\frac{33}{68}\)
\(A=\frac{11}{17}\)
A = \(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)\(\frac{4}{2.5}+\frac{4}{5.8}+\frac{4}{8.11}+.......+\frac{4}{65.68}\)\(\)4/2x5 + 4/5x8 + 4/8x11 + ....... + 4/65x68
Nhân cả 2 vế với 3/4 ta có :
3/4A = \(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)\(\frac{4}{2.5}+\frac{4}{5.8}+\frac{4}{8.11}+.......+\frac{4}{65.68}\)\(\)3/2x5 + 3/5x8 + 3/8x11 + ....... + 3/65x68
3/4A = 1/2-1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ........ + 1/65 - 1/68
3/4A = 1/2 - 1/68
3/4A = 33/68
A = 33/68 : 3/4
A = 11/17
A = 4/2x5 + 4/5x8 + .... + 4/65x68
Nhân cả hai vế với 3/4 ta có :
3/4A = 3/2x5 + 3/5x8 + .... + 3/65x68
3/4A = 1/2-1/5+1/5-1/8+....+1/65-1/68
3/4A = 1/2 - 1/68
3/4A = 33/68
A = 33/68 : 3/4
A = 11/17
