Question

Solve the equation cos x= -0.4 on the interval 0 less than or equal to x less than 2pi

Answer 1

Hello, danielleg,
It just so happens that 2pi is the period of the cosine function. The cosine, like all five of the other trig functions, is (+) in Quadrant 1. In which Quadrants is the cosine (-) (negative)?
You might want to draw a triangle within each quadrant in which the cosine is negative. Label the hypotenuse, the adjacent side and the opposite side. Can you figure out a way to determine what the angle is, in each case? Hint: Use the inverse cosine function.