Sylvie needs to solve $\cos(x)=−0.3$ on a math test. She knows that $\cos^{−1}(x)$ is the inverse function for cosine, so she applies this function to both sides of the equation. This gives her $x=\cos^{-1}(−0.3)\approx1.875$. But when her test is returned, she learns she earned only partial credit on the problem.  

1. Why did Sylvie only get partial credit?

   Answer (a):

   There are multiple solutions.

2. What should Sylvie have answered to receive full credit? Explain your answer.

   Answer (b):

   Sylvie needs to include all the solutions, which she can get using a graph or unit circle.