# Trig: Help!

• Mar 23rd 2013, 08:31 PM
dwee0309
Trig: Help!
Attachment 27674
Some questions I need help with, out of a selection of others. I've tried them, and have been scribbling all over my notepads, and just can't get it! I hope if someone helps with part a) for both of these questions, I'll be able to do b) c) and d) myself. Thanks! One of the main things i don't get in the second question is how cos(theta)= 2/3 in quadrant 4... I'm kind of new to this Math, but haven't had trouble so far with any previous questions leading up to this. And for the 1st question, I tried to find cos(x) and cos(y), then substituting into sin(x+y)=sin(x)cos(y)+sin(y)cos(x)... Apparently giving me the wrong answer (these questions are marked online, so maybe my syntax is wrong, but I'd also appreciate if anyone could check my answer/teach me how to do it...)

for 2: ((Headbang)i think wrong): 4(sqrt(5))/9
Not sure if u can see pics/questions..
• Mar 24th 2013, 01:01 AM
Prove It
Re: Trig: Help!
Yes 1. is wrong. You need to remember that $\displaystyle \sin{(2\theta)} = 2\sin{(\theta)}\cos{(\theta)}$. You have $\displaystyle \cos{(\theta)}$, so use the Pythagorean Identity to get $\displaystyle \sin{(\theta)}$ (remembering that it will be negative because you are in the fourth quadrant), and then finding $\sin{(2\theta)}$ will be a piece of cake.

Number 2. is also wrong. You need to use $\displaystyle \sin{(a + b)} = \sin{(a)}\cos{(b)} + \cos{(a)}\sin{(b)}$. To get the cosine values, use the Pythagorean Identity.
• Mar 24th 2013, 01:54 AM
Debsta
Re: Trig: Help!
Do you have your answers in the wrong order? The answer you give for Q2 is very close to the answer for the first question on your post. Remember sin (theta) is neg in 4th quadrant, so your answer should be the neg of what you have.

• Mar 24th 2013, 01:54 AM
dwee0309
Re: Trig: Help!
Sorry mixed up the answers. What i wrote for 2, was meant for 1, & vice versa: (I did use those identities)
1. 4(-sqrt(5))/9
2. (-(sqrt(7))/8)+(3(sqrt(3))/8)

They're still probably wrong though... I keep getting the same answer again and again though . I'll try again.
• Mar 24th 2013, 01:56 AM
Debsta
Re: Trig: Help!
They're correct...what's the problem? (You could simplify your second answer by adding the fractions with a common denominator.
• Mar 24th 2013, 01:57 AM
dwee0309
Re: Trig: Help!
Quote:

Originally Posted by Debsta
Do you have your answers in the wrong order? The answer you give for Q2 is very close to the answer for the first question on your post. Remember sin (theta) is neg in 4th quadrant, so your answer should be the neg of what you have.

Yeah just realised! Sorry! And thanks, must be the way I wrote it in my syntax assignment that's giving me wrong answers then :/ I'll try the first question again
• Mar 24th 2013, 01:58 AM
dwee0309
Re: Trig: Help!
I'm not so sure. If they're both correct, must just be the system, it's telling me it's incorrect. Might ask a lecturer tomorrow. Thanks
• Mar 24th 2013, 05:53 PM
dwee0309
Re: Trig: Help!
Hi everyone! It was definitely the form the maple syntax program accepted. The answers accepted were:

Q1: (-(sqrt(80)))/9
Q2: (((sqrt(27)))-((sqrt(7))))/8

Thanks for everyone who replied/messaged me. It really helped me understand further!