CPM Homework Help : CCG Problem 5-46

Estelle is trying to find $x$ in the triangle at right. She lost her scientific calculator, but luckily her teacher told her that $\sin23º\approx0.391$ , $\cos23º\approx0.921$ , and $\tan23º\approx0.424$ . Homework Help ✎

triangle

Write an equation that Estelle could use to solve for $x$ .

Step 1(a):

Use the Triangle Angle Sum Theorem to calculate the missing angle, this means all the angles in a triangle total $180º$ .

$67º + 90º + θ = 180º$
$−157 −157$
$θ = 23º$

Step 2(a):

Place a " $y$ " on the missing side. Write the trigonometric ratios for the angle, reference the Math Note box in Lesson 5.1.2 for extra help.

$\sin23º=\frac{y}{x}\ \ \ \ \cos23º=\frac{18}{x}\ \ \ \tan23º=\frac{y}{18}$

Hint (a):

Which equation can be used to solve for $x$ ?

Answer (a):

${\cos 23º=\frac{18}{x}}$

Without a calculator, how could Estelle find $\sin67º$ ? Explain.

Step 1(b):

$\text{sin}(67º)=\frac{\text{opposite}}{\text{hypotenuse}}=\frac{18}{x}$

Step 2(b):

$\text{sin}(67º)=\frac{18}{x}=\text{cos}(23º)$

Step 3(b):

$\cos23º\approx0.921$

Answer (b):

$\sin67º=\cos23º\approx0.921$