(1) 2（3xy-2x²-3y²)+3(x²-5xy+3y²)
= 6xy-4x²-6y²+3x²-15xy+9y²
= 6xy-15xy-4x²+3x²+9y²-6y²
=-9xy-x²+3y²
(2) 3(x²-2x+1)-4(2x²-x-3)
= 3x²-6x+3-8x²+4x+12
= 3x²-8x²-6x+4x+3+12
= -5x²-2x+15
(3) -(-a²+2ab+5²)+(-a²-ab+b²）,其中a=1/30,b=20
=a²-2ab-5²-a²-ab+b²
=a²-a²-2ab-ab+b²-5²
=-3ab+b²-25
=-3*1/30*20+20²-25
=-2+400-25
=373