\(A=\frac{1}{3\times5}+\frac{3}{5\times11}+\frac{2}{11\times15}+\frac{3}{15\times21}\)
\(A=\frac{1}{2}\times\left(\frac{2}{3\times5}+\frac{6}{5\times11}+\frac{4}{11\times15}+\frac{6}{15\times21}\right)\)
\(A=\frac{1}{2}\times\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+\frac{1}{15}-\frac{1}{21}\right)\)
\(A=\frac{1}{2}\times\left(\frac{1}{3}-\frac{1}{21}\right)\)
\(A=\frac{1}{2}\times\frac{2}{7}\)
\(A=\frac{1}{7}\)
A = \(\frac{1}{3.5}+\frac{3}{5.11}+\frac{2}{11.15}+\frac{3}{15.21}\)
A = \(\frac{1}{2}.\left(\frac{2}{3.5}+\frac{6}{5.11}+\frac{4}{11.15}+\frac{6}{15.21}\right)\)
A = \(\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+\frac{1}{15}-\frac{1}{21}\right)\)
A = \(\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{21}\right)\)
A = \(\frac{1}{2}.\frac{2}{7}\)
A = \(\frac{1}{7}\)
