How to create a Fraction equation with '[]' while numerator and denominator may have more than two lines: The new equation is:
\begin{equation}
q_{2n}^*=\Bigg[
-\frac{(\alpha -1) \alpha A_0^2 \beta k^2 \left(\lambda _h-\lambda
_l\right) \left(-c_l \left(\lambda _h-1\right)+c_h \left(\lambda _l-
1\right)+\left(c_n+c_{\text{nature}}-1\right) \left(\lambda _h-\lambda
_l\right)\right)+2 A_0 k \left(\alpha (\beta -1)+\alpha \beta c_h
\left(\lambda _h+1\right)+\alpha \beta \lambda _h \left(-2 c_n-2
c_{\text{nature}}-\lambda _h \delta _m+\delta _m+1\right)-(\alpha -1) c_l
\left(\lambda _l+1\right)+\lambda _l \left(-\alpha +2 (\alpha -1) c_n+2
(\alpha -1) c_{\text{nature}}+\alpha \left(-\delta _m\right)+\delta
_m\right)+\lambda _l+(\alpha -1) \lambda _l^2 \delta _m+1\right)+4
\left(c_n+c_{\text{nature}}-\delta _m\right)}{2 k \left(A_0 k \left(\alpha
(\beta -1)+\alpha \beta \lambda _h^2-2 \alpha \beta \lambda _h-(\alpha
-1) \left(\lambda _l-2\right) \lambda _l+1\right)+4\right)}
\Bigg]
\end{equation}
It hopes that the above equation could be look like this which created by mathtype: