TeXworks ⇒ why this code for formula is wrong?

[yaozhao]

Sample question:
If the long-run total cost curve for each firm is given by $TC=1000+100Q-10$Q^2$+$Q^3$$

[yaozhao]

I want to type:

TC=1000+100Q-10Q^2+Q^3

[Stefan Kottwitz]

$ is the symbol to start or to end a math environment. In the code above, the math mode was ended and then started and ended within the same formula. Here is how it should be done:

$TC=1000+100Q-10Q^2+Q^3$

Stefan