mathstrignometry
posted by anoynomous .
Prove that
tan1 (1/7) + tan1 (1/13) = tan1(2/9)

mathstrignometry 
Reiny
let tanA = 1/7 and tanB = 1/13
then tan(A+B) = (tanA + tanB)/( 1  tanAtanB)
= (1/7 + 1/13)/(1  (1/7)(1/13) )
= (20/91) / (90/91) = 20/90 = 2/9
and if we let tanC = 2/9
then A + B = C
thus tan^1 (1/7) + tan^1 (1/13) = tan^1 (2/9)
check with your calculator
take 2nd Tan (1/7) = 8.13010..
take 2nd Tan(1/13) = 4.39871
add them: 12.5288..
take 2nd Tan(2/9) = 12.5288..
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