20 17 the answer to winter vacation homework's "Algebraic Addition and Subtraction Ⅱ" in the first volume of seventh grade mathematics is the winter vacation homework reference answer for the first grade mathematics published by People's Education Press. Answer: a 1 A 2 B 3 C 4 C 5 D 6 D

Two 1-+

2 ( 1) n- 1，n，n+ 1 (2) 2n-4，2n-2，2n

3 -2b 4 5 27 6 89

three

1 、( 1)3( 1-2ab 2)-2(a2 b-4ab 2)(2)x-(3x-2)+(2x-3)；

Solution: Original formula =3-6ab2-2a2b+8ab2 Solution: Original formula = x-3x+2+2x-3.

=3-6ab2+8ab2-2a2b = x-3x+2x+2-3

= 3+(-6ab 2+8ab 2)-2a2b =(x-3x+2x)+(2-3)

=3+2ab2-2a2b = - 1

(3) 3a2-2(2a2+a)+2(a2-3a), where a =-2;

Solution: Original formula =3a2-4a2-2a+2a2-6a

=3a2-4a2+2a2-2a-6a

=(3a2-4a2+2a2)+(-2a-6a)

= a2-8a

Substituting a=-2, the original formula is =(-2)2-8×(-2).

= 4+ 16

= 20

2. Nitrogen

3、 10y+x 10x+y