2/1x2x3+2/2x3x4+2/3x4x5+.2/8x9x10=
=2{1/2[1/1*2-1/2*3]+1/2[1/2*3-1/3*4]+1/2[1/3*4-1/4*5]+...+1/2[1/8*9-1/9*10] }
=44/90
=22/45
2/1x2x3+2/2x3x4+2/3x4x5+.2/8x9x10=
=2{1/2[1/1*2-1/2*3]+1/2[1/2*3-1/3*4]+1/2[1/3*4-1/4*5]+...+1/2[1/8*9-1/9*10] }
=44/90
=22/45