(1)2a 5·(﹣a) 2﹣(﹣a 2) 3·(﹣7a),
=2a 7﹣7a 7
=﹣5a 7;
(2)(4x 2y+5xy﹣7x)﹣(5x 2y+4xy+x),
=﹣x 2y+xy﹣8x;
(3)(
x 2y﹣2xy+y 2)·3xy
=
x 3y 2﹣6x 2y 2+3xy 3;
(4)(4x 3y﹣6x 2y 2+12xy 3)÷(2xy),
=2x 2﹣3xy+6y 2;
(5)(x+2y) 2﹣(x+y)(x﹣y),
=x 2+4xy+4y 2﹣x 2+y 2,
=4xy+5y 2,
x=2,y=
时,原式=4×2×
+5×
=
.