p.17 問4

(1)	$$ \begin{align} (x-2)(x-6) &= x^2-6x-2x+12 \\ &= x^2-8x+12 \end{align}$$
(2)	$$ \begin{align} (x-4)(x+5) &= x^2+5x-4x-20 \\ &= x^2+x-20 \end{align}$$
(3)	$$ \begin{align} (2a+1)(a+4) &= 2a^2+8a+a+4 \\ &= 2a^2+9a+4 \end{align}$$
(4)	$$ \begin{align} (3x+5)(4x-7) &= 12x^2-21x+20x-35 \\ &= 12x^2+x-35 \end{align}$$

p.17 問5

(1)	$$ \begin{align} (3a+2b)(2a+3b) &= 6a^2+9ab+4ab+6b^2 \\ &= 6a^2+13ab+6b^2 \end{align}$$
(2)	$$ \begin{align} (9a-2b)(5a+6b) &= 45a^2+54ab-10ab-12b^2 \\ &= 45a^2+44ab-12b^2 \end{align}$$
(3)	$$ \begin{align} (7x+4y)(x-5y) &= 7x^2-35xy+4xy-20y^2 \\ &= 7x^2-31xy-20y^2 \end{align}$$
(4)	$$ \begin{align} (2x-3y)(8x-y) &= 16x^2-2xy-24xy+3y^2 \\ &= 16x^2-26xy+3y^2 \end{align}$$

p.17 問6

(1)	$$ \begin{align} (a+1)(a+b-1) &= a^2+ab-a+a+b-1 \\ &= a^2+ab+b-1 \end{align}$$
(2)	$$ \begin{align} (a+2b)(2a+b+1) &= 2a^2+ab+a+4ab+2b^2+2b \\ &= 2a^2+5ab+2b^2+a+2b \end{align}$$
(3)	$$ \begin{align} (x+2y-1)(2x-y) &= 2x^2-xy+4xy-2y^2-2x+y \\ &= 2x^2+3xy-2y^2-2x+y \end{align}$$
(4)	$$ \begin{align} (x-y+3)(3x-2y) &= 3x^2-2xy-3xy-2y^2+9x-6y \\ &= 3x^2-5xy-2y^2+9x-6y \end{align}$$