Answer:
The law of cosines
- c² = a² + b² + - 2ab cosC
Step-by-step explanation:
When triangle is obtuse, the cos C is negative. Suppose the three sides are a = 5, b = 6, and c = 10. Then the law of cosines says 100 = 25 + 36 – 60 cos C, so cos C = - 49/60 = - 0.81667. 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. You’ll get C = 2.2556 radians = 129.237°.
actually, the explanation of that is long. I just shorten it
