5年级奥数题，100分酬谢！简便运算

1、[51(2/3)]/(3/5)+[71(3/4)]/(4/7)+[91(5/4)]/(5/9)
=(50+5/3)/(3/5)+(70+7/4)/(4/7)+(90+9/5)/(5/9)
=30+1+40+1+50+1
=123
2 第一步先第一项和最后一项提取公因式然后化简

24又1/19减1/17）乘以（1/7加1/8加1/9）减4乘以（1/7减1/8加1/9）加（1/7加1/8加1/9）乘以（1/17减1/19）
=(1/7+1/8+1/9)(4又1/19-1/17+1/17-1/19)-4(1/7-1/8+1/9
=4/7+4/8+4/9-4/7+4/8-4/9
=1
哈哈

解
1原题=(50+5/3)/(5/3)+(70+7/4)/(4/7)+(90+9/5)/(5/9)
=50/5/3+5/3/3/5+70/7/4+7/4/4/7+90/5/9+9/5/5/9
=50*3/5+1+70*4/7+1+90*5/9+1
=30+1+40+1+50+1
=123
2原题=[4(1/19)-1/17]*(1/7+1/8+1/9)-[4(1/7+1/8+1/9)+(1/7+1/8+1/9)*(1/17-1/19)
=(1/7+1/8+1/9)*[4(1/19)-1/17-4+1/17-1/19] 注释：提取 公因式小学也叫乘法结合率
=(1/7+1/8+1/9)*0
=0

1原题=(50+5/3)/(5/3)+(70+7/4)/(4/7)+(90+9/5)/(5/9)
=50/5/3+5/3/3/5+70/7/4+7/4/4/7+90/5/9+9/5/5/9
=50*3/5+1+70*4/7+1+90*5/9+1
=30+1+40+1+50+1
=123
2原题=[4(1/19)-1/17]*(1/7+1/8+1/9)-[4(1/7+1/8+1/9)+(1/7+1/8+1/9)*(1/17-1/1