已知x^2+9y^2=9,x+3y=5,求xy
(x+3y)²=x²+9y²+6xy=9+6xy=25;
6xy=16;
xy=8/3;
已知2x-y=3,xy=2,求(x+1/2y)^2
(x+y/2)²
=(2x+y)²/4
=[(2x-y)²+8xy]/4
=[9+2×8]/4
=25/4;
已知a^2-a-1=0,求a^4+1/a^4的值
a-1-1/a=0;
a-1/a=1;
a²+1/a²=(a-1/a)²+2=1+2=3;
a^4+1/a^4=(a^2+1/a^2)^2-2=9-2=7;
如果本題有什么不明白可以追問,