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  4. cos θ . cos(90 - θ) - sin θ sin (90 - θ) equals:

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Q. $\cos \theta .\cos(90 - \theta) - \sin \theta \sin (90 - \theta)$ equals:

Trigonometric Functions

Solution:

Consider
$\cos \theta . \cos(90 - \theta) - \sin \, \theta \, \sin(90 - \theta)$
$= \cos \theta . \sin \, \theta - \sin \, \theta . \cos \, \theta = 0$