(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)(
1
2 x 2y-2xy+y 2)•3xy
=
3
2 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,
当x=2,y=
1
2 时,原式=4×2×
1
2 +5×
1
4 =
21
4 .