=-[1/(1x3)+1/(2x4)+1/(3x5)+1/(4x6)+1/(5x7)]
=-1/2[1-1/3+1/2-1/4+1/3-1/5+1/4-1/6+1/5-1/7]
=-1/2[1+1/2-1/6-1/7]
=-25/42