Question

先化简，后求值

(2a-1)2-2(a-1)(2a+3)-7.其中a=1/2

Answer 1

原式=4a2-4a+1-4a2-2a+6-7
=-6a
=-6×1/2
=-3

Answer 2

(2a-1)2-2(a-1)(2a+3)-7
=4a²-4a+1-2(2a²+a-3)-7
=4a²-4a+1-4a²-2a+6-7
=-6a

带入 a=1/2 得
=-3

Answer 3

(2a-1)2-2(a-1)(2a+3)-7
=4a-2-(2a-2)(2a+3)-7
=4a-2-{2a(2a+3)-2(2a+3)}-7
=4a-2-{4a^2+6a-4a-6}-7
=4a-2-4a^2-6a+4a+6-7
=2a-4a^2-3
=1-1-3
=-3

Answer 4

(2a-1)2-2(a-1)(2a+3)-7
=4a-2-(2a-2)(2a+3)-7
=4a-2-(4a^2+6a-4a-6)-7
=4a-2-4a^2-2a+6-7
=-4a^2+2a-3
=-4(1/2)^2+2*1/2-3
=-1+1-3
=-3

Answer 5

解：(2a-1)2-2(a-1)(2a+3)-7
=4a-2 -[2(2a^2+3a-2a-3)]-7
=4a-2-4a^2-6a+4a+6-7
=2a-4a^2-3
将a=1/2带入得
2*1/2-4*1/2^2-3
＝-3