error in mathematical expression

Question

I type in the code shown below for a displayed formula. However the output is not what it should be. Can you tell me what is the mistake over here?

$$ \frac{(1-a)\frac{$q_{f}$}{{$q_{n}$}^a}}{$p_{n}$} \cdot $q_{f}$ \cdot $\Delta p$ $$

Answer 1

First of All, You can't nest $ in $$. Therefore your formula should be like

$$ \frac{(1-a)\frac{q_{f}}{{q_{n}}^a}}{p_{n}} \cdot q_{f} \cdot \Delta p $$

Answer 2

Building on Hassan Zakeri's answer, you have two potential problems. As Hassan says, you can't nest.

But it is also much better not to use $$ to start display maths. Here's a better solution:

\[ \frac{(1-a)\frac{q_{f}}{{q_{n}}^a}}{p_{n}} \cdot q_{f} \cdot \Delta p \]

For further explanation, please read this question and its answers: Why is [ … ] preferable to $$?