Question

Medal will be awarded..please help

Answer 1

A is an acute angle and B is an obtuse angle, where sin A = \[\frac{ 4 }{ 5 }\] and cos B = \[\frac{ -3 }{ 5 }\]. Without finding the values of A and B, calculate cos3A

Answer 2

cos3A=4cos^A-3cosA

Answer 3

if you can find cosA then you are done

Answer 4

are you familiar with
sin^2A +cos^2A=1?
(4/5)^2+(cosA)^2 = 1
(cosA)^2 = 1 -16/25
(cosA)^2 = 9/25
cosA = +3/5 (because, since A is acute, cosA cannot have a negative value. So we left with only the positive)

Answer 5

now you know
cosA=3/5
and apply that to our previous equation of cos3A

Answer 6

cos3A=4cos^3A-3cosA
cosA=3/5

Answer 7

I am quite familiar with the formula

Answer 8

okay, Hope you can complete by yourself then

Answer 9

Can I have some help substiting cos A into the formula? Becuase I am having some trouble witht he cos3A bit

Answer 10

okay. I'll give you the final answer

Answer 11

4*(0.6)^3-3(0.6)

Answer 12

-0.936

Answer 13

Is that the final answer?

Answer 14

Isn't it supposed to be in degrees?

Answer 15

A can be written in degrees or radians
but cosine of any angle is always a real number between -1 an 1 including them

Answer 16

Thank you!

Answer 17

Question, how come you changed from cos^A to (cosA)^2?