1*2*3分之2加2*3*4分之2加3*4*5分之2加到11*12*13分之2等于多少
问题描述：
1*2*3分之2加2*3*4分之2加3*4*5分之2加到11*12*13分之2等于多少
答
2x[( 1/1-1/2-1/3)+(1/2-1/3-1/4)+(1/3-1/4-1/5)+.+(1/11-1/12-1/13)]
=2x[ 1/1-1/2-1/3+1/2-1/3-1/4+1/3-1/4-1/5+1/4+.+1/11-1/12-1/13]
=2x[1-1/3-1/4-1/5-1/6-1/7-1/8-1/9-1/10/1/11-1/12-1/13]
=2x[1+1/2-(1/2+1/3+1/4+1/5+1/6+1/7+1/8+1/9+1/10+1/11+1/12+1/13]
=2x[1+1/2-1]
=2x1/2
=1