Question

Which quadrants should be considered when solving `cosx=(sqrt2)/4` for `0^@<=x<=360^@`, and why?

Answer 1

First and fourth.

Explanation:

You should look for the quadrants in which the cosine is positive. Since the cosine is the `x`-projection of the points in the unit circle, you must look for the quadrants which points have positive `x`-coordinates, and they are the first and the fourth (visually, the top right and bottom right ones).