2/2×4 + 2/4×6 + 2/6×8 + 2/8×10 + 2/10×12
=1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10+1/10-1/12
=1/2-1/12
=6/12-1/12
=5/12