A community for students.
Here's the question you clicked on:
 0 viewing
lilsis76
 3 years ago
suppose sin theta = 1/5 and 0<theat< pi/2
compute cos theta
and compute sin 2theta...
i have these formulas given but i dont know what to do with them. let me draw them
lilsis76
 3 years ago
suppose sin theta = 1/5 and 0<theat< pi/2 compute cos theta and compute sin 2theta... i have these formulas given but i dont know what to do with them. let me draw them

This Question is Closed

phi
 3 years ago
Best ResponseYou've already chosen the best response.1i have these formulas given... The expressions you posted are specific examples, not formulas. To answer this question suppose sin x = 1/5 and 0<x< pi/2 (Let me use x, it's easier to type than theta) First, recall the definition of sin: opposite over hypotenuse. 2nd, it is helpful to draw a graph. dw:1350682184808:dw

phi
 3 years ago
Best ResponseYou've already chosen the best response.1they tell us x is between 0 and pi/2 (90º) so we know x is in the first quadrant. the cos x is adjacent over hypotenuse. we know the hypotenuse (it's 5) use pythagorean theorem to find the bottom leg of the right triangle (that will be the adjacent side we need for the cos) can you do that?

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0yes let me try it on a paper.

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0a= sqrt 24 > i can reduce giving me a=sqrt 4 sqrt 6 a= 2sqrt6

phi
 3 years ago
Best ResponseYou've already chosen the best response.1yes, use a's value to find \[ \cos(x)= \frac{2\sqrt{6}}{5}\]

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0sorry about that, this thing is being slow, this openstudy site

phi
 3 years ago
Best ResponseYou've already chosen the best response.1to find compute sin 2x I remember sin(a+b)= sin(a)cos(b)+cos(a)sin(b) if a and b are the same=x, we can write this as sin(x+x) = sin(x) cos(x) + cos(x) sin(x) sin(2x)= 2 sin(x) cos(x) so replace sin(x) with 1/5 and cos(x) with 2sqrt(6)/5 in that formula and simplify to get sin(2x)

phi
 3 years ago
Best ResponseYou've already chosen the best response.1being slow Yes, sometimes it acts up. Some days are better than others.

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0okay ill try to find the first one, but how did you get \[2\sqrt{6} / 5 ? < how did u get the 5?\]

phi
 3 years ago
Best ResponseYou've already chosen the best response.1the cos x is adjacent over hypotenuse

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0how did u get the 5? haha sorry i cant read the other writting either. and i did sin 2x= sin 2(1/5)  that would make it sin 2/5 right?

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0okay i see the cos =5 now. is 2/5 right?

phi
 3 years ago
Best ResponseYou've already chosen the best response.1no. it is not the simple (otherwise it would not be math) first, what do you have for sin(x) and cos(x)?

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0sin x is opp/hyp= 1/5 cos x is adj/hyp= 2sqrt{6}/5

phi
 3 years ago
Best ResponseYou've already chosen the best response.1ok, that is good. I repeat my post from above to find compute sin 2x I remember sin(a+b)= sin(a)cos(b)+cos(a)sin(b) if a and b are the same=x, we can write this as sin(x+x) = sin(x) cos(x) + cos(x) sin(x) sin(2x)= 2 sin(x) cos(x) so replace sin(x) with 1/5 and cos(x) with 2sqrt(6)/5 in that formula and simplify to get sin(2x)

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0in the sin(a+b)= sin(a)cos(b)+cos(a)sin(b)? like sin( 1/5 + 2sqrt6/5) = sin 1/5 cos 2sqrt6/5 + cos 1/5 sin 2sqrt6/5

phi
 3 years ago
Best ResponseYou've already chosen the best response.1no. don't get confused. I was showing that the "sum of angles" formula simplifies to sin(2x)= 2 sin(x) cos(x) For example, if x were 30º, you could use this formula to find the sin(60º) sin(60)= sin(2*30)= 2 sin(30) cos(30) in this case, you are asked to find the sin(2x). You know sin(x) and cos(x). Can you find sin(2x)?

phi
 3 years ago
Best ResponseYou've already chosen the best response.1sin x is opp/hyp= 1/5 cos x is adj/hyp= 2sqrt{6}/5 replace sin(x) with 1/5 replace cos(x) with 2sqrt{6}/5 in the formula sin(2x)= 2 sin(x) cos(x)

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0okay let me try it. i am trying to find it on my paper

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0sin 2x = 2 sin 1/5 cos 2sqrt 6 / 5

phi
 3 years ago
Best ResponseYou've already chosen the best response.1be careful x is not 1/5 sin(x) is 1/5 try again.

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0sin 2x = 2sin 1/5 cos x

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0thats all im understanding

phi
 3 years ago
Best ResponseYou've already chosen the best response.1when we say sin x =1/5 and then write, for example cos(x) sin(x) we can replace sin(x) with 1/5 (that is what = means) so cos(x) sin(x) could be written as cos(x) * 1/5 can you use that idea in sin(2x)= 2 sin(x) cos(x) when sin x= 1/5 cos x= 2sqrt{6}/5

phi
 3 years ago
Best ResponseYou've already chosen the best response.1in other words, replace sin x with 1/5 and replace cos(x) with 2sqrt(6)/5

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0into the formula you gave me? the...sin(2x)= 2 sin(x) cos(x) ? what is that equation for also?

phi
 3 years ago
Best ResponseYou've already chosen the best response.1yes, in the formula sin(2x)= 2 sin(x) cos(x) I'll post a video that explains this, so you can watch it when you have time.

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0like this: 2(1/5) = 2(1/5)(2sqrt6 / 5) ?

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0HEYY?! i just realized, did we solve the compute cos theta? was that the 2sqrt6 / 5?

phi
 3 years ago
Best ResponseYou've already chosen the best response.1closer. we know sin(x) and cos(x). but we do not know sin(2x) we start with sin(2x)= 2 sin(x) cos(x) replace sin(x) with 1/5 sin(2x)= 2* 1/5 * cos(x) replace cos(x) with 2sqrt6/5 sin(2x)= 2* 1/5 * 2sqrt6/5 notice that sin(2x) is not 2*1/5

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0will the 2 (1/5) = 2/5? or is that wrong?

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0i ddont know how to mult. fractions.

phi
 3 years ago
Best ResponseYou've already chosen the best response.1yes 2*1/5 is 2/5 the answer is \[ \sin(2\theta)= \frac{4\sqrt{6}}{25} \] (I used theta again just to match the problem)

phi
 3 years ago
Best ResponseYou've already chosen the best response.1Here is the first video on trig http://www.khanacademy.org/math/trigonometry/v/basictrigonometry

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0oh....okay, thank you ill watch the video. but one quesiton. why dont we multiply the sqrt by the 2 also?

phi
 3 years ago
Best ResponseYou've already chosen the best response.1Here is a list of videos on fractions http://www.khanacademy.org/math/arithmetic/fractions I would watch the first few just to see if they make sense. Later on, if you run into a problem, go to this site and search for a video that explains it.

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0okay. thank you so much. imma watch a few of these then get back to the homework.

phi
 3 years ago
Best ResponseYou've already chosen the best response.1\[ \sin(2x)= 2\cdot \frac{1}{5} \cdot \frac{ 2\sqrt{6}}{5} \] to multiply fractions you multiply top*top and bottom*bottom (whole numbers are "tops") \[ \sin(2x)= \frac{2\cdot 1 \cdot 2\sqrt{6}}{5\cdot 5} \] that simplifies to \[ \sin(2x)= \frac{4\sqrt{6}}{25} \]

lilsis76
 3 years ago
Best ResponseYou've already chosen the best response.0oh...okay now i see it, dang lol i never saw it like that before. well thank you I shall watch the videos and ill be back later
Ask your own question
Sign UpFind more explanations on OpenStudy
Your question is ready. Sign up for free to start getting answers.
spraguer
(Moderator)
5
→ View Detailed Profile
is replying to Can someone tell me what button the professor is hitting...
23
 Teamwork 19 Teammate
 Problem Solving 19 Hero
 Engagement 19 Mad Hatter
 You have blocked this person.
 ✔ You're a fan Checking fan status...
Thanks for being so helpful in mathematics. If you are getting quality help, make sure you spread the word about OpenStudy.