Take the 2-minute tour ×
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.

This is my attempt:

$(\sin \theta+1)(\sin \theta-1) = \sin\theta^2 - \sin\theta + \sin\theta - 1$

$= \sin^2\theta - 1$

$= -\cos^2\theta$

Is it correct, and can it be improved? Thanks!

share|improve this question
1  
Looks good. Nice work. –  John Habert Feb 3 at 5:13
1  
Pretty much as short as it gets. –  user61527 Feb 3 at 5:14
1  
It is absolutely correct. –  rah4927 Feb 3 at 5:14
    
Similarly,you can simplify $(\cos\theta+1)(\cos\theta-1)$ –  rah4927 Feb 3 at 5:16
    
Two things - First: You've written $\sin\theta^2$. This should actually (in the context of the question) be $\sin^2\theta$. Second: It is correct, but the step $sinθ^2−sinθ+sinθ−1$ is not required. In fact, you could just use the identity $(a+b)(a-b)=a^2-b^2$. –  SDevalapurkar Feb 3 at 5:40

1 Answer 1

Yes this is correct.You must be knowing that $(x+y)(x-y)=x^2-y^2$. Therefore $(sin\theta+1)(sin\theta-1)=sin^2\theta-1=-cos^2\theta$

share|improve this answer

Your Answer

 
discard

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.