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The cosine of an acute angle is equal to the sine of its complement.
sin 72^(∘)
Keep in mind that complementary angles add up to 90^(∘). To find the complement of 18, we subtract our given angle from 90. Now, we can rewrite cos 18^(∘) in terms of sine. cos 18^(∘) = sin(90^(∘)-18^(∘)) = sin 72^(∘)
