Left aligning a {align*} with spacing?

Question

I'm using align, and I would like the equations to be left aligned, but when I write a single & to left align, they are placed right beside the \Leftrightarrow.

How can I fix this?

Here's my code:

\begin{align*}
& \left(\dfrac{a}{2}\right)^2\cdot\pi+\left(\dfrac{b}{2}\right)^2  
\cdot\pi-\left(\dfrac{\sqrt{a^2+b^2}}{2}\right)^2\cdot\pi+ab &=ab\\
\Leftrightarrow & \dfrac{a^2}{4}\cdot\pi+\dfrac{b^2}{4}\cdot\pi-\dfrac  
{a^2+b^2}{4}\cdot\pi+ab &=ab\\
\Leftrightarrow & \dfrac{a^2}{4}\cdot\pi+\dfrac{b^2}{4}\cdot\pi-\left(\dfrac
{a^2}{4}\cdot\pi+\dfrac{b^2}{4}\cdot\pi\right)+ab &=ab\\
\Leftrightarrow &  ab&=ab
\end{align*}

Answer 1

I think this is what you want, using alignat. Note an align(at) environment with n alignment points requires 2n – 1 &, not n.

\documentclass[x11names]{article}
\usepackage{amsmath}

\begin{document}

\begin{alignat*}{2}
&\biggl(\dfrac{a}{2}\biggr)^2\cdot\pi+\left(\dfrac{b}{2}\right)^2
\cdot\pi-\left(\dfrac{\sqrt{a^2+b^2}}{2}\right)^2\cdot\pi+ab & &=ab\\
\Leftrightarrow{} &\dfrac{a^2}{4}\cdot\pi+\dfrac{b^2}{4}\cdot\pi-\dfrac
{a^2+b^2}{4}\cdot\pi+ab & &=ab\\
 \Leftrightarrow{} & \dfrac{a^2}{4}\cdot\pi+\dfrac{b^2}{4}\cdot\pi-\left(\dfrac
{a^2}{4}\cdot\pi+\dfrac{b^2}{4}\cdot\pi\right)+ab & &=ab\\
\Leftrightarrow{}&ab& & =ab
\end{alignat*}

\end{document}