（1）计算（3a-5b-4c)^2
=9a²+25b²+16c²-30ab-24ac+40bc;
(2)计算(3x-2y+1)(2x+3y-1)
=6x²+9xy-3x-4xy-6y²+2y+2x+3y-1
=6x²-6y²+5xy-x+5y-1;
(3)因式分解(2x-3y)^2-3(2x-3y)(2x+3y)
=(2x-3y)(2x-3y-6x-9y)
=(2x-3y)(-4x-12y)
=-4(2x-3y)(x+3y);
（4）若4x^3+ay^3与2/1xy^b-2是同类项,求2a-b的值
∴a+3=1;
b-2=3;
∴a=-2;
b=5;
∴2a-b=-4-5=-9;
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