Answer
$-1$
Work Step by Step
$\dfrac {\tan 80+\tan 55}{1-\tan 80\tan 55}=\tan \left( 80+55\right) =\tan 135=-1$
Precalculus (6th Edition)
by Lial, Margaret L.; Hornsby, John; Schneider, David I.; Daniels, Callie
Published by
Pearson
ISBN 10:
013421742X
ISBN 13:
978-0-13421-742-0
Chapter 7 - Trigonometric Identities and Equations - 7.3 Sum and Difference Identities - 7.3 Exercises - Page 679: 61
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