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1.解(1)(x-3)(2x+5)=2x²+5x-6x-15=2x²-x-15.
(2)(2a+1)(-a-2)=-2a²-4a-a-2=-2a²-5a-2.
(3)(x+1/2)(x-1/3)=x²-1/3x+1/2x-1/6=x²+1/6x-1/6.
(5)(xy+1)(xy-4)=x²y²-4xy+xy-4=x²y²-3xy-4.
(6)(5m-4n)(4m-5n)=20m²-25mn-16mn+20n²=20m²-41mn+20n².
2.解:(1)(2a-b)(a+2b-3)=2a²+4ab-6a-ab-2b²+3b=2a²+3ab-6a+3b-2b²;
(2)(x+y+5)(x+y+4)=[(x+y)+5][(x+y)+4]=(x+y)²+9(x+y)+20=x²+xy+xy+y²+9x+9y+20=x²+2xy+9x+9y+y²+20.
3.解:(x-1)(2x+1)-2(x-5)(x+2)=2x²-x-1-2(x²-3x-10)=2x²-x-1-2x²+6x+20=5x+19.当x=1/5时,5x+19=5×1/5+19=20.
4.解:(90+a)(60+a)-90×60=90×60+90a+60a+a²-90×60=(a²+150a)(cm²).
答:它的面积将增加(a²+150a)cm².