a/ Đề bài sai, \(21⋮̸10\Rightarrow21^{10}⋮̸10\) mà \(200⋮10\Rightarrow21^{10}⋮̸200\)
b/ \(A=4-\left(x^2-4x+4\right)=4-\left(x-2\right)^2\le4\)
\(A_{max}=4\) khi \(x=2\)
c/ Từ đề bài ta có:
\(\left\{{}\begin{matrix}f\left(-1\right)=6\\f\left(2\right)=3\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}-a+b+1=6\\2a+b+4=3\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}-a+b=5\\2a+b=-1\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=-2\\b=3\end{matrix}\right.\) \(\Rightarrow y=x^2-2x+3\)
d/ \(A=x^2-3x=x^2-3x+\frac{9}{4}-\frac{9}{4}=\left(x-\frac{3}{2}\right)^2-\frac{9}{4}\ge-\frac{9}{4}\)
\(A_{min}=-\frac{9}{4}\) khi \(x=\frac{3}{2}\)
