# Thread: Trig & Pythagoras Triples

1. ## Trig & Pythagoras Triples

Hi, got this question and I've got the answer=9/19, but the answer given in the book is 63/16.

I am using the identity Tan(A+B)=(TanA+TanB)/(1-[TanATanB]). The question is as follows;

Given that A and B are acute angles such that sinA=5/13 and cosB=3/5, find the exact value of Tan(A+B).

I've got as far as the identity (shown above) and have TanA=5/12 and TanB=4/3, using pythagoras triples. However when I put this into the identity I get completly confused. Can someone check that my values for TanA and TanB are correct and show me how to complete the question. Thanks in advance everyone.

2. Yes, you're correct. tanA = 5/12 and tanB = 4/3.

You got lost in your simplifying the expression with all those fractions.

tan(A+B) = [5/12 +4/3] /[1 -(5/12)(4/3)]
= [(5 +16)/12] /[1 -20/36]
= [21/12] /[1 -5/9]
= [21/12] /[4/9]
= [21/12]*[9/4]
= [21*9] /[12*4]
= [21*3] /[4*4]
= 63/16