Cant align

\varphi =\frac { 1+\sqrt { 5}}{ 2 } =1.6180339887...\\
\varphi =\frac { a+b }{ a } =\frac { a }{ b }

renders like

How can I align left

If you haven't already solved this, try:

\begin{align*}
\varphi &= \frac { 1+\sqrt { 5}}{ 2 } =1.6180339887...\\ 
\varphi &= \frac { a+b }{ a } =\frac {a}{ b }
\end{align*}

Try

\\
\varphi =\frac { 1+\sqrt { 5}}{ 2 } =1.6180339887...\\
\varphi =\frac { a+b }{ a } =\frac { a }{ b }

Both solution work. Can you explain why \ in the beginning works?