Hi, hello, so, I am having problems on sinosoidal equations. I have watched the videos but they dont seem to correspond to me? An example is 7cos(9x)-1=1. I know to isolate until I get cos(9x)=2/7. I dont know what to do afterwards to solve the problem. If anyone could help, that would be greatly appreciated!!
Here's the solution:
7cos(9x) -1 = 1
cos(9x) = 2/7 like you said
9/x = cos^-1(2/7)
9/x = 73.40
x = 9/73.40
x = 0.1226
I used cos^-1 to solve the problem, which you should be able to find on a scientific calculator. That lets you move the cos to the other side of the equation. For more of an in-depth explanation to solving sinusoidal equations, see the videos and practice in this section: https://www.khanacademy.org/math/trigonometry/trig-equations-and-identities/basic-sinusoidal-equations/v/sine-solutions.
The third line of the above solution should be:
9x = cos^-1(2/7) (9 times x instead of 9 divided by x)
Then:
9x = 73.40 degrees or 1.281 radians
x = 8.16 (8.15555...) degrees or 0.142 (0.1423333...) radians
Plugging either of these two answers in, with your calculator's mode set correctly, verifies the original problem.
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