1）原式=1/3*（1-1/4+1/4-1/7+1/7-1/10+1/2-1/5+1/5-1/8+1/8-1/11+1/3+1/6+1/6-1/9+1/9-1/12）
=1/3*（1-1/10+1/2-1/11+1/3-1/12）
=1/3*（9/10+9/22+1/4）
=1/3*243/220
=81/220
2）原式=2*[1/(2*3)+1/(3*4)+1/(4*5)+……+1/(50*51)]
=2*(1/2-1/3+1/3-1/4+1/4-1/5+……+1/50-1/51)
=2*(1/2-1/51)
=2*49/102
=49/51
3）原式=(3*2)/(1*4)*(4*3)/(2*5)*(5*4)/(3*6)*(6*5)/(4*7)*……*(51*50)/(49*52)
=3/1*2/4*4/2*3/5*5/3*4/6*6/4*……*51/49*50/52
=3*50/52
=75/26