Question

Answer 1

Answer 2

\(\rightarrow\dfrac{1}{2}C=\dfrac{1}{2}\left(\dfrac{6}{15.18}+\dfrac{6}{18.21}+...+\dfrac{6}{87.90}\right)\\ \dfrac{1}{2}C=\dfrac{3}{15.18}+\dfrac{3}{18.21}+...+\dfrac{3}{87.90}\\ \dfrac{1}{2}C=\dfrac{1}{15}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{21}+...+\dfrac{1}{87}-\dfrac{1}{90}\\ \dfrac{1}{2}C=\dfrac{1}{15}-\dfrac{1}{90}=\dfrac{1}{18}\\ \rightarrow C=\dfrac{1}{18}:\dfrac{1}{2}\\ C=\dfrac{1}{9}\)