The law of cosines - Math Central

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Quandaries & 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.

Chetna,

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.

Penny

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