a) \(\left(2x+1\right)^2-\left(2x-7\right)^2\)
\(=\left(2x+1+2x-7\right)\left(2x+1-2x+7\right)\)
\(=8\left(4x-6\right)\)
\(=8\cdot2\left(2x-3\right)\)
\(=16\left(2x-3\right)\)
Thay x=-5 vào ta có:
\(16\cdot\left(2\cdot-5-3\right)=-208\)
c) \(\left(x+9\right)^2-\left(x-8\right)^2-\left(32x-1\right)\)
\(=x^2+18x+81-\left(x^2-16x+64\right)-32x+1\)
\(=x^2+18x+81-x^2+16x-64-32x+1\)
\(=2x+18\)
Thay x=\(\dfrac{1}{2}\) vào ta có:
\(2\cdot\dfrac{1}{2}+18=19\)
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