Adding line break in displayed math, only one equation [duplicate]

This question may seem like a duplicate, but I haven't seen any satisfactory answer in similar questions.

Is there a way to add a line break in displayed math mode without using multiple-line environments such as gather or eqnarray? I want to display an equation that spans multiple lines inside an equation environment (so that the entire thing only gets one number).

My current code:

$$A_i=A_0-\sum_{j=0}^{i-1}[(2^i\cdot3)\cdot\frac{s_i^2}{18}\cdot sin(\alpha)]=% \sum_{j=0}^{i-1}[(2^i\cdot3)\cdot\frac{(\frac{s_0}{3^i})^2}{18}\cdot sin(180º-\frac{120º}{2^i})]=% \sum_{j=0}^{i-1}[(2^i\cdot3)\cdot\ddfrac{s_0^2}{18\cdot3^{2i}}\cdot sin(180º-\frac{120º}{2^i})]$$


This "overflows" the line:

I want to add a line break in between (after one of the = signs), but still have the whole thing labelled as (4.7).