Chapter 13, Trigonometric Ratios and Functions - 13.5 Apply the Law of Sines - 13.5 Exercises - Skill Practice - Page 886: 1

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To use the law of sines, we are making a ratio, $sin(A)/a=sin(B)/b$. Thus we must know 3 out of 4 of these numbers and that means we must know both the angle measure and side length opposite that angle, in addition to another angle or side of the triangle.