Splitting a long equation with numerator and denominator over more lines

Question

I tried to split a long equation with numerator and denominator over more lines in different ways (How can I split an equation over two (or more) lines and How to wrap a long equation in Latex), but they do not work (latex gives me error). How can I solve the problem?

The expression (produced by Mathematica) is the following (the splits should occur where there are exponential terms):

y(t) = h(t) = \frac{e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}-\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} k_1 -e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}+\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} k_1-e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}-\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} k_2+e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}+\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} k_2-e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}-\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} k_{12}+e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}+\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} k_{12}+e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}-\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} k_{21}-e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}+\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} k_{21}+e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}-\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1
   k_2+k_{21} k_2+k_1 k_{12}\right)}+e^{t
   \left(-\frac{k_1}{2}-\frac{k_2}{2}-\frac{k_{12}}{2}-\frac{k_{21}}{2}+\frac{1}{2
   } \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)}\right)} \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1
   k_2+k_{21} k_2+k_1 k_{12}\right)}}{2
   \sqrt{\left(k_1+k_2+k_{12}+k_{21}\right){}^2-4 \left(k_1 k_2+k_{21} k_2+k_1
   k_{12}\right)} V_1} 

Thank you for your help.

Answer 1

I hope I counted well:)

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
   A&:=\sqrt{(k_1+k_2+k_{12}+k_{21})^2-4(k_1k_2+k_{21}k_2+k_1k_{12})}\\
   B&:=k_1+k_2+k_{12}\\
   C&:=k_1-k_2-k_{12}\\
a(t)&:=\exp\left(-\frac{A+B}{2}t\right)\\
b(t)&:=\exp\left(\frac{A-B}{2}t\right)\\
y(t)&=h(t)=\frac{a(t)(A+C+k_{21})+b(t)(A-C)-k_{21}}{2AV_1} 
\end{align*}
\end{document}