1/2*4+1/4*6+1/6*8+.1/48*50
=1/2(1/2-1/4)+1/2(1/4-1/6)+1/2(1/6-1/8)+.1/2(1/48-1/50)
=1/2(1/2-1/4+1/4-1/6+...+1/48-1/50)
=1/2(1/2-1/50)
=6/25