Trig Proofs / write an equation using only sines, cosines

[EndersCatalyst]

Instructions: Write an equation using only sines and cosines.
sin3x + cos2x I don't know what to do because it's already an expression using sines and cosines.

Prove that the following is true by working only on the left-hand side. (So you can't start from the right).
sin3x = (sinx)(3-4 sin^2x)
Work so far: sinx (sin2x)

I would appreciate any help. The teacher is not at school in the mornings to help.

 

[Denis]

The teacher is not at school in the mornings to help.

Huh? Is he/she there in the afternoons?
What do you know about sines and cosines?
Did you try and find out using google?

> Work so far: sinx (sin2x)
Why are you calling that "work so far"?

 

[EndersCatalyst]

No, she's not there in the afternoons. I tried to get out of one of my classes to go to her class and talk to her, but couldn't leave.
We've been doing Identities in PreCal lately and I have 3 pages of Id's but still cannot figure out the problems. Yes, I tried to get help using Google but didn't find much.
I called it "work so far" because that's all of the work I can think to do. Anything else I think of doesn't work out right.That was my first step and now I'm stuck.

 

[Denis]

No, she's not there in the afternoons. I tried to get out of one of my classes to go to her class and talk to her, but couldn't leave.
We've been doing Identities in PreCal lately and I have 3 pages of Id's but still cannot figure out the problems. Yes, I tried to get help using Google but didn't find much.
I called it "work so far" because that's all of the work I can think to do. Anything else I think of doesn't work out right.That was my first step and now I'm stuck.

If all that is true, then I'm flabbergasted; a teacher that can't be reached :shock:

Sines, cosines and the likes definitely can only be "taught" in a clasroom environment; impossible here.

Is everybody else in your class in the same boat?
If so, why don't you get together and "protest" to whoever is in charge at this weird school of yours...

 

[fasteddie65]

sin 3x + cos 2x = 3 sin x - 4 sin^3 x + cos^2 x - sin^2 x

 

[Deleted member 4993]

Instructions: Write an equation using only sines and cosines.
sin3x + cos2x I don't know what to do because it's already an expression using sines and cosines.

Prove that the following is true by working only on the left-hand side. (So you can't start from the right).
sin3x = (sinx)(3-4 sin^2x)
Work so far: sinx (sin2x) <<< What is this? Are you trying to say this is equal to sin(3x)? That would be incorrect.

sin (3x) = sin (x + 2x)

Now apply the formula

sin (A + B) = sin(A) * cos(B) + cos(A) * sin(B)

also

cos(2D) = 1 - 2 * sin[sup:39q4jsc1]2[/sup:39q4jsc1](D)

Now continue....
I would appreciate any help. The teacher is not at school in the mornings to help.

 

[Denis]

sin3x = (sinx)(3-4 sin^2x)
Work so far: sinx (sin2x)

Well, that was "generous" of you guys, but if this poor kid thinks that SIN(3x) = SIN(x)SIN(2x),
then he needs a teacher; no exotic equations will presently help him.
My opinion of course; and I respect yours: but if it ain't same as mine, then it's wrong :lol:

 

[EndersCatalyst]

Ok, I'm so sorry for wasting you time, Denis. I can't help it if I haven't had a decent math teacher in a while. A lot of people in my school don't know basic algebra because some teachers are inadequate and don't teach. I was only asking for a bit of help. If someone, possibly you, had been nice enough to help me out just a little bit, I could have completed the problem. You don't need to be so rude about it. BTW, I'm a she.

Thanks to all of the posters who actually DID try to help me. I appreciate it.