十字相乘法分解因式:
(1)x2+3x+2
(2)x2-3x+2
(3)x2+2x-3
(4)x2-2x-3
(5)x2+5x+6
(6)x2-5x-6
(7)x2+x-6
(8)x2-x-6
(9)x2-5x-36
(10)x2+3x-18
(11)2x2-3x+1
(12)6x2+5x-6.