You can use it in a couple of ways. First, if you know one angle and the two adjacent sides, then you can determine the opposite side. Second, if you know all three sides of a triangle, then you can use it to find any angle. As you can see in the graphs on the previous page, the cosine of an obtuse angle is negative. This is fine, and your calculator will compute the arccosine properly.
C c Note that the law of sines says that three ratios are equal.
Like the law of cosines, you can use the law of sines in two ways. First, if you know two angles and the side opposite one of them, then you can determine the side opposite the other one of them. Second, if you know two sides and the angle opposite one of them, then you can almost determine the angle opposite the other one of them. This situation is indeterminant. Knowing two sides and the angle opposite one of them is not always enough to determine the triangle.