已知a=4x^2-5xy+y^2,b=x^2+3xy-5y^2.求a-2b的值,求-2a+3b的值

解
a-2b
=(4x²-5xy+y²)-2(x²+3xy-5y²)
=(4x²-2x²)+(-5xy-6xy)+(y²+10y²)
=2x²-11xy+11y²
-2a+3b
=-2(4x²-5xy+y²)+3(x²+3xy-5y²)
=(-8x²+3x²)+(10xy+9xy)+(-2y²-15y²)
=-5x²+19xy-17y²