x^2+y^2-2x+4y+5
=(x^2-2x+1)+(y^2+4y+4)
=(x-1)^2+(y+2)^2
=0
且(x-1)^2≥0,(y+2)^2≥0
所以(x-1)^2＝0,(y+2)^2＝0
解得：x=1,y=-2
2x-3y
=2+6
=8