Math CentralQuandaries & Queries


Question from chetna, a parent:

Q 1) In triangle LMN, l=7, m=5 , n=4. find ANGLE N.

After applying the rule and substituting values i'm getting
Cos n= 58/40. Is there something wrong. The answer at the back of the book is 34 degrees.


I got

n2 = m2 + l2 - 2 m l cos(N) so
42 = 52 + 72 - 2(5)(7) cos(N)

which gives

cos(N) = 58/70

and thus N = 34 degrees.


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