Last minute explanations on Trig Identities.
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Elumas (
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June 14th, 2010
I’m ready for my Math Final…. except there are several things regarding Trig Identities that just don’t process for me. I picked several sample problems from my practice packet that just don’t click. If you can help me work through them it would be greatly appreciated:
1. What needs to be changed in the last two lines to make this correct?
line one: cos2x=.5
line two: 2x={ 30º+360nº
{ 180º-30º+360nº
line three: x={ 15º+360nº
{ 75º+360nº
2. solve 2sinxcosx=cosx where x is [-π, π]
3. solve tan2θ=-1.2 where 0<θ<360
4. (1-cotθ)^2=csc^2θ-2cotθ
5. (sinθ+cosθ)/cosθ-(sinθ-cosθ)/sinθ=secθcscθ
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3 Answers
I don’t know how to do 1–3 and 5 or maybe I forgot, but I can help with 4.
THE ZERO WITH THE BELT ACROSS=X
4. (1-cotX)^2 = csc^2 X -2cotX
(1-cotX)(1-cotX) = csc^2 X -2cotX
1-cot -cot +cot^2 X = csc^2 X -2cotX
You rearrange the first part of the equation.
(1+ cot^2 X) -cot -cot = (csc^2 X) -2cotX
Csc^2 X -2 cotX = csc^2 X -2cotX
Also, did you take notes? Can you use your notes or cheat-sheets on your final? You can always wake up early tomorrow to ask your teacher for help or stay after school…
You have the right idea for 1, but cos 60 = .5.
For 2, just divide both sides by cosx to get 2sinx=1. You should be able to handle it from there.
For 3, first find arc tan of 1.2. That gives 2 theta. Then divide by 2 to get theta.
4 was done by @cyndihugs
For 5, start by expanding the divisions to get
sin/cos + 1 – 1 + cos/sin = sin/cos + cos/sin
The common denominator is sincos, giving
(sin^2+cos^2))/sincos = 1/sincos
Let me add:
for 1, remember to divide your 360n by 2 as well.
for 2, cosx could be 0, and in that case you can’t divide it out. And when is cosx=0? That’ll give you another solution.
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