Hi just worked through this question:
Solve the equation:
I get cos(x) = 1/3
and x = +- 70.5
The answer says it's also +- 180 which makes sense but i wondered how this second result can be found?
As you have written it you are right but the book's answer suggest your limits are actually . This means are allowable
You are cancelling out sin(x) in the second step which is potentially equal to 0. Instead, solve this equation via factorising.
Either: (which is where -180,0,180 comes from)
or which you did
It is also interesting to note that this equation has a vertical asymptote at