Problem with equation within 'multline' environment

Question

\begin{multline}
   [H_{s,u}]=\sum_{n=1}^{N}\alpha_{n}\sqrt{g_{t}(\phi_{n},\theta_{n},\theta_{tilt})}\\
   \times exp(ik\rho(4\lambda(M-1)/M)\cos(\phi_{n}-\varphi_{s})\sin\theta_{n})\\
   \times \sqrt{g_{r}(\varphi_{n},\vartheta_{n})exp(ik\rho(4\lambda(n-1)/M)\cos(\phi_{n}-\varphi_{s})\sin\theta_{n})
\end{multline}

Whenever I type this equation, it gives me the following error

Runaway argument?
[H_{s,u}]=\sum _{n=1}^{N}\alpha _{n}\sqrt {g_{t}(\phi _{n},\theta _{\ETC.

 Paragraph ended before \multline was complete.
    <to be read again> 
                       \par 

Kindly help me

Answer 1

\begin{multline}
[H_{s,u}]=\sum_{n=1}^{N}\alpha_{n}\sqrt{g_{t}(\phi_{n},\theta_{n},\theta_{tilt})}\\
\times exp(ik\rho(4\lambda(M-1)/M)\cos(\phi_{n}-\varphi_{s})\sin\theta_{n})\\
\times \sqrt{g_{r}(\varphi_{n},\vartheta_{n})exp(ik\rho(4\lambda(n-1)/M)\cos(\phi_{n}-\varphi_{s})\sin\theta_{n})
\end{multline}

is missing a } for the square root on the last line, note also exp should be \exp. and tilt should be \mathrm{tilt}

Answer 2

Some other improvements using aligned nested in equation, using two rows and and aligning on the right side:

\documentclass{article}
\usepackage[showframe]{geometry}
\usepackage{mathtools}

\begin{document}

\begin{equation}
\begin{aligned}
[H_{s,u}]= \sum_{n=1}^{N} \alpha_{n}\sqrt{g_{t}(\phi_{n},\theta_{n},\theta_\mathrm{tilt})}
\times \exp(ik\rho(4\lambda(M-1)/M)\cos(\phi_{n}-\varphi_{s})\sin\theta_{n})\\
\times \sqrt{g_{r}(\varphi_{n},\vartheta_{n})\exp(ik\rho(4\lambda(n-1)/M)\cos(\phi_{n}-\varphi_{s})\sin\theta_{n})}
\end{aligned}
\end{equation}

\end{document}