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 (10th Edition)
by Lial, Margaret L.; Hornsby, John; Schneider, David I.; Daniels, Callie
Published by
Pearson
ISBN 10:
0321671775
ISBN 13:
978-0-32167-177-6
Chapter 2 - Acute Angles and Right Triangles - Section 2.3 Finding Trigonometric Function Values Using a Calculator - 2.3 Exercises - Page 64: 51
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