Sign up ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

$$ \Rightarrow (\sec^2 \theta) \frac{d \theta}{dt} = \frac{A \frac{dB}{dt} - B \frac{dA}{dt}}{A^2} $$ $$ \Rightarrow \frac{d \theta}{dt} = (\cos^2 \theta) \left(\frac{A \frac{dB}{dt} - B \frac{dA}{dt}}{A^2} \right) $$

I have this. One question. I think I should divide by $\sec^2 \theta$. But I saw this solution to the problem. Why does sec become cos?

share|cite|improve this question
Because $\sec(\theta) = \frac{1}{\cos(\theta)}$ by definition. –  Sasha Nov 18 '11 at 16:42

1 Answer 1

up vote 2 down vote accepted

By definition $\sec \theta = \frac{1}{\cos \theta}$, so it disappears from the left-hand side when you multiply by the cosine.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.