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Verified Answer :

MissPhiladelphia

A common mistake would be to apply distributive property to make it cos 90 - cos theta. However, this is not applicable because these are trigonometric functions. Similarly, you need to apply trigonometric identities/properties

cos(A - B) = cosAcosB + sinaAsinB

In analogy,

cos (90 - ∅) = cos90cos∅ + sin90sin∅

Since cos 90 = 0; sin 90 = 1,

cos (90 - ∅) = sin∅