1. EXAMPLE 1 Rewrite a formula with two variables Solve the formula C = 2πr for r. Then find the radius of a circle with a circumference of 44 inches. SOLUTION C = 2 π r C 2π = r STEP 1 Solve the formula for r. STEP 2Substitute the given value into the rewritten formula. Write circumference formula. Divide each side by 2π. r = C 2π = 44 2π 7 Substitute 44 for C and simplify. The radius of the circle is about 7 inches. ANSWER

2. GUIDED PRACTICE for Example 1 Find the radius of a circle with a circumference of 25 feet. 1. SOLUTION C = 2 π r C 2π = r STEP 1 Solve the formula for r. STEP 2Substitute the given value into the rewritten formula. Write circumference formula. Divide each side by 2π. r = C 2π = 25 2π 3.9 Substitute 25 for C and simplify. The radius of the circle is about 4 feet. ANSWER

3. GUIDED PRACTICE for Example 1 The formula for the distance d between opposite vertices of a regular hexagon is d = where a is the distance between opposite sides. Solve the formula for a. Then find a when d = 10 centimeters. 2. 2a2a 3 SOLUTION d3 a = 2 35 When d = 10cm, a = cm or 8.7cm