EXAMPLE 2 Find the area of a regular polygon DECORATING You are decorating the top of a table by covering it with small ceramic tiles. The table top is.

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Presentation transcript:

EXAMPLE 2 Find the area of a regular polygon DECORATING You are decorating the top of a table by covering it with small ceramic tiles. The table top is a regular octagon with 15 inch sides and a radius of about 19.6 inches. What is the area you are covering? SOLUTION STEP 1 Find the perimeter P of the table top. An octagon has 8 sides, so P = 8(15) = 120 inches.

EXAMPLE 2 STEP 2 So, QS = (QP) = (15) = 7.5 inches To find RS, use the Pythagorean Theorem for RQS. a = RS – = Find the apothem a. The apothem is height RS ofPQR. BecausePQR is isosceles, altitude RS bisects QP. Find the area of a regular polygon

EXAMPLE 2 STEP 3 Find the area A of the table top. 1 2 A = aP Formula for area of regular polygon (18.108)(120) 1 2 Substitute Simplify. Find the area of a regular polygon So, the area you are covering with tiles is about square inches. ANSWER

EXAMPLE 3 Find the perimeter and area of a regular polygon A regular nonagon is inscribed in a circle with radius 4 units. Find the perimeter and area of the nonagon. SOLUTION 360° The measure of central JLK is, or 40°. Apothem LM bisects the central angle, so m KLM is 20°. To find the lengths of the legs, use trigonometric ratios for right KLM. 9

EXAMPLE 3 sin 20° = MK LK sin 20° = MK 4 4 sin 20° = MK cos 20° = LM LK cos 20° = LM 4 4 cos 20° = LM The regular nonagon has side length s = 2MK = 2(4 sin 20°) = 8 sin 20° and apothem a = LM = 4 cos 20°. Find the perimeter and area of a regular polygon

EXAMPLE 3 Find the perimeter and area of a regular polygon So, the perimeter is P = 9s = 9(8 sin 20°) = 72 sin 20° 24.6 units, and the area is A = aP = (4 cos 20°)(72 sin 20°) 46.3 square units ANSWER

GUIDED PRACTICE for Examples 2 and 3 3. Find the perimeter and the area of the regular polygon. about 46.6 units, about units 2 ANSWER

GUIDED PRACTICE for Examples 2 and Find the perimeter and the area of the regular polygon. 70 units, about units 2 ANSWER

GUIDED PRACTICE for Examples 2 and units, about units 2 ANSWER Find the perimeter and the area of the regular polygon.

GUIDED PRACTICE for Examples 2 and 3 6. Which of Exercises 3–5 above can be solved using special right triangles? Exercise 5ANSWER