# Math Help - Help~Trigo Funstion~!!

1. ## Help~Trigo Funstion~!!

1.Cos2A+cos4A-cos3A=0, for 0≤A≤360
2.Show that Cos^-1(A)=Sin^-1 square root (1-A^2), (Inverse Trigonometric Function)
3.2cos3A-sin3A=Square root 5, for 0≤A≤360
4.Cos(A-∏/6) + sin (∏/6-A)=1, for 0≤A≤360
5.Tan3A=cot(∏/3-A), for 0≤A≤360

Thx for who helpING!!

2. Please show your work. What have you tried and why do you think it wasn't working?

3. I stuck at beginning.How to start?

4. 1) Copy down the problem.
2) Examine a list of known identities for relationships that may prove useful.

This one can help...

cos(a+b) = cos(a)cos(b) - sin(a)sin(b)

3) You dive in and try something. Paralysis never solve problems.
4) You look for symmetries so you don't have to do the same work twice.

Hint: With a little effort, you can get the first one to:

$8\cos^{4}(a)-4\cos^{3}(a)-6\cos^{2}(a)+3\cos(a)=0$

If you convert this to look more familiar to the Algebra in you...

$8x^{4}-4x^{3}-6x^{2}+3x=0$

You should be able to solve this by factoring and the Quadratic Formula. (Don't forget your Rational Root Theorem!)