Triangle ABC is shown in the figure below. The
measure of ∠A is 40°, AB = 18 cm, and AC = 12 cm. Which of the following is the length, in centimeters, of BC?
I have the answer, but I don't understand it.

Question

Triangle ABC is shown in the figure below. The
measure of ∠A is 40°, AB = 18 cm, and AC = 12 cm. Which of the following is the length, in centimeters, of BC?
I have the answer, but I don't understand it.

anonymous · Answer

The answer is $${\sqrt{12^{2}+18^{2}-(2)(18)(12)\cos 40}}$$

hartnn · Answer

heard of Cosine rule?

anonymous · Answer

Yes, but I dont know how to apply it

hartnn · Answer

http://www.mathsisfun.com/algebra/trig-cosine-law.html draw your triangle on the paper first :)

hartnn · Answer

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like that ^^

now name the lengths and angles