## Thinking Mathematically (6th Edition)

$b \approx 5$ cm
RECALL: In a right triangle, $\sin{A} = \dfrac{\text{length of opposite side}}{\text{length of hypotenuse}} \\\cos{A} = \dfrac{\text{length of adjacent side}}{\text{length of hypotenuse}} \\\tan{A} = \dfrac{\text{length of opposite side}}{\text{length of adjacent side}}$ Use the cosine formula to obtain: $\cos{A} = \dfrac{\text{length of adjacent side}}{\text{length of hypotenuse}} \\\cos{72^o}=\dfrac{b}{15}$ Multiply 15 to both sides of the equation to obtain: $\cos{72^o} \cdot 15 = b \\4.635254916= b \\5 \approx b$ Thus, $b \approx 5$ cm.