1/(11*13*15) =1/4 * [1/(11*13) - 1/(13*15)]1/11*13*15+1/13*15*17+1/15*17*19+……+1/27*29*31=1/4 * [1/(11*13) - 1/(13*15)]+ 1/4 * [1/(13*15) - 1/(15*17)]+ 1/4 * [1/(15*17) - 1/(17*19)] + ...+1/4 * [1/(...