Question 674533
Derive the identity for tan(a-b)using tan(a-b)=tan[a+(-b)]. 
After applying the formula for the tangent of the sum of two angles, use the fact that the tangent is an odd function.
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if function is even, f(-x)=f(x)
if function is odd, f(-x)=-f(x)
tan, being an odd function:
tan(-x)=-tan(x)
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Identity: tan(a+b)=(tana+tanb)/(1-tana tanb)
tan[(a+(-b)]=(tana+tan(-b))/(1-tana tan(-b))
tan(-b)=-tan b
tan(a-b)=(tana-tanb)/(1+tana tanb)