7)x^3+y^3=(x+y)(x^2-xy+y^2)=(x+y)[(x+y)^2-3xy] 得到xy=2/9
x^6+y^6=(x^3+y^3)^2-2(xy)^3=65/729
8) a^2+1/a^2=(a-1/a)^2+2*a*(1/a)=(a-1/a)^2+2=3
所以 a-1/a=1或-1
9) x^3+y^3=(x+y)[(x+y)^2-3xy]=100 得到xy=30
所以 x^2+y^2=(x+y)^2-2xy=100-2*30=40
10) x^2+y^2=(x+y)^2-2xy 得到xy=-0.5
x^6+y^6=(x^2)^3+(y^2)^3=(x^2+y^2)[(x^2+y^2)^2-3x^2y^2=6.5
1).(3x-y+5z)²=9X^2+y^2+25z^2-6xy+30xz-10yz 公式: (a+b+c)²=a²+b²+c²+2ab+2bc+2ca
2)。|a|=|b|=|c|
3)用待定系数法
9)40
本来打算给你解答的,进来一看,额,好多,其实我比楼主还懒
(3x-y+5z)²=(3x-y)²+25z²+2*(3x-y)*5z=9x²+y²-6xy+25z²+30xz-10yz
(x-5y-9)²=(x-5y)²+81-2*(x-5y)*9=x²+25y²-10xy+81-18x+90y