2x2-6xy+9y2-4x+5
=(x²-6xy+9y²)+(x²-4x+5)
=(x²-6xy+9y²)+(x²-4x+4)+1
=(x-3y)²+(x-2)²+1≥1＞0
所以,2x2-6xy+9y2-4x+5的值是正数.