\(\hept{\begin{cases}\sqrt{1+x}=a\\\sqrt{4-x}=b\end{cases}}\)
\(\Rightarrow\hept{\begin{cases}a+b+ab=5\\a^2+b^2=5\end{cases}}\)
\(\Leftrightarrow\hept{\begin{cases}2a+2b+2ab=10\\a^2+b^2=5\end{cases}}\)
\(\Rightarrow\left(a+b\right)^2+2\left(a+b\right)=15\)
\(\Leftrightarrow\orbr{\begin{cases}a+b=3\\a+b=-5\end{cases}}\)