In what's below, the equation numbering is annoying and forcing it to move up would be a solution. Anyway what are your suggestions to fix this without modifying the equations?
\documentclass[11pt]{book}
\usepackage[latin1]{inputenc}
\usepackage[a4paper,top=3.5cm,bottom=3cm,left=3.6cm,right=3.6cm]{geometry}
\usepackage{lipsum}
\usepackage{amsmath}
\begin{document}
\lipsum[4]
\begin{equation}\begin{aligned}
ac_1 &= a_1c_1 \\
a^2c_2 &= a_1c_2 + a_2c_1^2 \\
a^3c_3 &= a_1c_3 + 2a_2c_1c_2 + a_3c_1^3 \\
a^4c_4 &= a_1c_4 + a_2(c_2^2+2c_1c_3) + 3a_3c_1^2c_2 + a_4c_1^4 \\
a^5c_5 &= a_1c_5 + a_2(2c_1c_4+2c_2c_3) + a_3(3c_1^2c_3+3c_1
c_2^2) + a_4(c_1^3c_2+3c_1^2c_2) + a_5c_1^5 \end{aligned}
\end{equation}
\lipsum[4]
\end{document}