a) Thay y=8 vào \(\left(P\right):y=\frac{-x^2}{2}\):
\(8=\frac{-x^2}{2}\Rightarrow x=\pm4\)
Vậy M(4;8) hoặc (-4;8).
b) \(\frac{-x^2}{2}=x+m\)
\(\Leftrightarrow-x^2-2x-2m=0\)
\(\Leftrightarrow x^2+2x+2m=0\)
Để (d) cắt (P) tại 2 điểm pb thì Δ>0
\(\Rightarrow4-8m>0\Leftrightarrow m< \frac{1}{2}\)
Có: \(y_1=x_1+m;y_2=x_2+m\)
\(\Rightarrow\left(x_1+y_1\right)\left(x_2+y_2\right)=\frac{33}{4}\)
\(\Rightarrow\left(2x_1+m\right)\left(2x_2+m\right)=\frac{33}{4}\)
\(\Leftrightarrow4x_1x_2+2x_1m+2x_2m+m^2=\frac{33}{4}\)
\(\Leftrightarrow4x_1x_2+2m\left(x_1+x_2\right)+m^2=\frac{33}{4}\)
Theo hệ thức Vi-et: \(\left\{{}\begin{matrix}x_1+x_2=-2\\x_1x_2=2m\end{matrix}\right.\)
\(\Rightarrow8m-4m+m^2=\frac{33}{4}\)
\(\Leftrightarrow m^2+4m=\frac{33}{4}\)
\(\Leftrightarrow m^2+4m-\frac{33}{4}=0\)
\(\Leftrightarrow\left[{}\begin{matrix}m=\frac{3}{2}\left(KTM\right)\\m=\frac{-11}{2}\left(TM\right)\end{matrix}\right.\)
Vậy m=\(\frac{-11}{2}\) thỏa mãn.
