Cos(A-B), SInA=3/5 SinB=5/13

Cos(A-B), SinA=3/5 SinB=5/13

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Cos (A-B) = CosACosB + SinASinB

Since we're dealing with "Pythagorean Triples" triangles, CosA = 4/5 and CosB = 12/13

So we have.....(4/5)(12/13) + (3/5)(5/13) =  48/65 + 15/65 = 63/65