error in mathematical expression

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?

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

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I removed the thanks (just the style here). Thanks is best expressed by upvoting helpful answers. –  qubyte May 19 '12 at 12:28

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 $$ - Great, thanks!! – Abhimanyu Arora May 19 '12 at 12:41 Building on Hassan Zakeri's answer, you have two potential problems. As Hassan says, you can't nest. But it is also much better not to use $$ 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$\$?