Math CentralQuandaries & Queries


Question from JAMIE, a student:

a triangle with a side(b)37m an angle(C)70degrees and (a)79m find values of angles A and B and length of side c

Hi Jamie.

Use the law of cosines: c2 = a2 + b2 - 2abCos(C)

This lets you calculate c. Then you can use the other form to find A:
a2 = b2 + c2 - 2bcCos(A)

You will need to use inverse cosine (arc cos) to solve for angle A.

To find B, you could use the law of cosines again, as you did to find A, but a shortcut is to recall that the three angles of any triangle add up to 180 degrees, so it would be faster just to subtract: B = 180 - A - C.

Stephen La Rocque

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