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March 5, 2015

March 5, 2015

Posted by **samuel** on Monday, April 18, 2011 at 7:59am.

- geometry -
**MathMate**, Monday, April 18, 2011 at 9:02amThe cosine rule gives the formula for the cosine of an angle when the three sides are known:

cos(A)=(B²+C²-A²)/(2BC)

where B and C are lengths of sides adjacent to A.

We know that a positive angle is acute when cos(A) is positive, which in turn requires that

B²+C²-A²>0 ....(1)

So if all three angles of a triangle have to be acute, we have the two additional requirement analogous to (1) above:

C²+A²-B²>0

A²+B²-C²>0

If the third side can be chosen such that all three conditions are satisfied, the triangle has acute angles.

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