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Big Ideas Math Geometry, 2014

BI

Big Ideas Math Geometry, 2014 View details

5. The Sine and Cosine Ratios

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Exercise 10 Page 498

The sine of an acute angle is equal to the cosine of its complement.

cos 9^(∘)

Practice makes perfect

We want to write sin 81^(∘) in terms of cosine. To do so, let's recall an important property of the sine and cosine of complementary angles.

The sine of an acute angle is equal to the cosine of its complement.

Keep in mind that complementary angles add up to 90^(∘). To find the complement of 81, we subtract our given angle from 90. Now, we can rewrite sin 81^(∘) in terms of cosine. sin 81^(∘) = cos(90^(∘)-81^(∘)) = cos 9^(∘)

Subchapter links

Communicate Your Answer p.493

Monitoring Progress p.495-497