简便计算1 （-6.25)的3次方X（5分之4）的7次方X 8的4次方 X （负一又4分之1）的8次方2 2000X19

（-6.25)^3X（4/5）^7X 8^4X （-5/4）^8
=-5^6/4^3X4^7/5^7X2^4X4^4X5^8/4^8
=-5^7X2^4
=-(5X2)^4X5^3
=-5^3X10^4
=-1250000
2000X19991999——1999X20002000
=2000X10001X1999-1999X2000X10001
=0
3/(x+1)+7/(y-3)=2,2/(x+1)-3/(y-3)=5/21
设u=1/(x+1),v=1/(y-3),则原式变形为：
3u+7v=2,2u-3v=5/21
u=1/3,v=1/7
x+1=3,y-3=7
x=2,y=10