\(z+1+2i=\left(1+i\right)\left|z\right|=\left|z\right|+i.\left|z\right|\)
\(\Leftrightarrow z=\left|z\right|-1+\left(\left|z\right|-2\right)i\)
Lấy mođun 2 vế:
\(\Rightarrow\left|z\right|=\sqrt{\left(\left|z\right|-1\right)^2+\left(\left|z\right|-2\right)^2}\)
\(\Leftrightarrow\left|z\right|^2=\left|z\right|^2-2\left|z\right|+1+\left|z\right|^2-4\left|z\right|+4\)
\(\Leftrightarrow\left|z\right|^2-6\left|z\right|+5=0\Rightarrow\left[{}\begin{matrix}\left|z\right|=1\left(l\right)\\\left|z\right|=5\end{matrix}\right.\)
\(\Rightarrow a^2+b^2=5\)
Không đủ dữ kiện để tính \(P=a+b\)