1/4+1/28+1/70+1/130+1/208
=1/1x4+1/4x7+1/7x10+1/10x13+1/13x16
=1/3x(1-1/4+1/4-1/7+....+1/13-1/16)
=1/3x(1-1/16)
=5/16
