(1)
（y+2)(y+4)+1
=y^2+6y+8+1
=y^2+6y+9
=(y+3)^2
(2)
(2a+1)(2a-1)+(a-2)²-4（a+1)(a-2)
= 4a^2 -1 +a^2-4a+4 - 4a^2+4a+8
=a^2+11
计算结果和 a^2有关,所以当a=2或者a=-2时,结果都是 15,是一样的.
(3)设a=3.456,则
原式
=a(a-1)(a+2) - a^3 -(a-2)^2
=a^3+a^2-2a -a^3 - a^2+4a-4
=2a-4
=2(a-2)
把 a = 3.456代入,得
原式
= 2(3.456-2)
=2*1.456
=2.912