第28页 - 通城学典课时作业本七年级数学苏科版江苏专用

C

$2a+1$

$2a^2−6a+25$

$(a+b)(a−b)=a^2−b^2$

$ \begin{aligned}解：原式&=(x²-1)(x²+1)({x}^4+1) \\ &=({x}^4-1)({x}^4+1) \\ &={x}^8-1 \\ \end{aligned}$

$ \begin{aligned}解：原式&=4a^2+4ab+b^2-4a^2+9b^2 \\ &=4ab+10b^2 \\ \end{aligned}$

$解：原式=4(x^2−2x+1)−(4x^2−9)$

$=4x^2−8x+4−4x^2+9$

$=−8x+13.$

$当x=\frac{1}{2}时，原式=−8×\frac{1}{2}+13=−4+13=9$

D