1/42+1/56+1/72+1/90+1/110+1/132=1/(6x7)+1/(7x8)+1/(8x9)+1/(9x10)+1/(10x11)+1/(11x12)=1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11+1/11-1/12=1/6-1/12=1/12