1.x^2+6x+y^2-8y+30
=(x+3)²+(y-4)²+5
当x=-3；y=4时,原式有最小值5
2.若x^2+2x-1=0
x²=-2x+1
那么5x^2+10x-3
=5(-2x+1)+10x-3
=-10x+5+10x-3
=2