21 問9(1)(x+y+3)(x+y-5)
= (A+3) (A-5)
= A²-2A-15
= (X+4)=2(x+4)=15
x²+12+2x-24-28-13
(?) (a+b+c)³
=(A+C)²
= A² + 2 Ac+c³
= a²+b²+2abc + C²
2
((3) (α-6-6)²
=(A-6)²
=A2-12A+36
= a²= b² + 12ab+36
(4)(a+b+3)(a-b+3)
= (a+b) (A-b)
= A²-6²
710 (1) (x-2)² + (x+4)(x+1)
=x4x+4+x²+5xt4
=2x+x+8
= a² = b² +9
(2)2(x+1)(x-1)-(x-3)(x+2)
=2(x²-1)-(x²-x-6)
: 2x² - 2 - x² + x + 6
=x+x+4