Answer
False
Work Step by Step
$\cos 40^{\circ} = 0.7660444431$
$1 - 2\sin^{2} 80^{\circ} = 1-1.939692621 = -0.939692621$
They are not equal, therefore false.
Trigonometry (11th Edition) Clone
by Lial, Margaret L.; Hornsby, John; Schneider, David I.; Daniels, Callie
Published by
Pearson
ISBN 10:
978-0-13-421743-7
ISBN 13:
978-0-13421-743-7
Chapter 2 - Acute Angles and Right Triangles - Section 2.3 Finding Trigonometric Function Values Using a Calculator - 2.3 Exercises - Page 67: 55
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