6.2 Gather
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\begin{gather}\label{gathereq}
\sum_{i=1}^{15} x^i = x^1 + x^2 + x^3 + x^4 + x^5 + x^6 + x^7
+ x^8 + x^9 + x^{10} + x^{11} + x^{12} + x^{13} + x^{14} + x^{15}\\
2^1 + 2^2 + 2^3 + 2^4 + 2^5 + 2^6 + 2^7 + 2^8 + 2^9 + 2^{10}
+ 2^{11} + 2^{12} + 2^{13} + 2^{14} + 2^{15}\nonumber
\end{gather}
\begin{gather} \label{gathereq} \sum _{i=1}^{15} x^ i = x^1 + x^2 + x^3 + x^4 + x^5 + x^6 + x^7 + x^8 + x^9 + x^{10} + x^{11} + x^{12} + x^{13} + x^{14} + x^{15}\\ 2^1 + 2^2 + 2^3 + 2^4 + 2^5 + 2^6 + 2^7 + 2^8 + 2^9 + 2^{10} + 2^{11} + 2^{12} + 2^{13} + 2^{14} + 2^{15}\nonumber \end{gather}And we can reference the equation using label and ref (2).
