A typical triangle has sides #a=5# and #b=1# and angle #C= pi/3#. Find #c# ?

1 Answer
Dean R.
May 11, 2018

By the Law of Cosines,

#c = sqrt{5^2 + 1^2 - 2(5)(1) (1/2) } = sqrt{21} #

Explanation:

I have a few thousand trig problems under my belt and it's startling and sad just how many use multiples of #30^circ# or #45^circ.# Question writers, the world has more than two triangles!

#pi/3# is of course #60^circ# and its cosine is #1/2#.

By the Law of Cosines,

#c^2 = a^2 + b^2 - 2 ab cos C#

#c = sqrt{5^2 + 1^2 - 2(5)(1) (1/2) } = sqrt{21} #

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