I cannot overemphasize how important “showing your work” is to your success in mathematics.
“Showing your work means, when any calculation is involved, writing down enough to let someone familiar with the course reproduce what you did. If a classmate or your instructor can tell how you reached your answer, without having to guess, you have shown your work”
“There are several good reasons [for showing your work]:
Showing your work in an organized way helps you organize your thoughts, which in turn makes you less likely to make a mistake.
On homework, usually you have answers available. If your answer doesn’t match the book’s answer, showing your work helps you figure out what you did wrong. It can also help a tutor figure out what you did wrong, if you go for help.
On tests, if you get a wrong answer your instructor can use your work to figure out what you didn’t understand, and point it out so that you avoid that mistake in the future.
On tests, if you get a wrong answer but your work shows you understood the core of the problem, most instructors will allow partial credit. A bare answer that is wrong has to count as a zero.”
All quoted text is from http://www.tc3.edu/instruct/sbrown/calc/showwork.htm and is reposted with permission from Stan Brown.