Answer:
CD = 12.7 cm
Step-by-step explanation:
[tex]\text{Use the Pythagorean theorem to calculate the length of the CB:}\\\\CB^2+BA^2=AC^2\\\\CB^2+6^2=12^2\\\\CB^2+36=144\qquad\text{subtract 36 from both sides}\\\\CB^2=108\to CB=\sqrt{108}\\\\CB=\sqrt{36\cdot3}\qquad\text{use}\ \sqrt{ab}=\sqrt{a}\cdot\sqrt{b}\\\\CB=\sqrt{36}\cdot\sqrt3\\\\CB=6\sqrt3[/tex]
[tex]\text{Use sine to calculate the length of CD:}\\\\sin\theta=\dfrac{opposite}{hypotenuse}\\\\\theta=55^o\\opposite=6\sqrt3\\hypotenuse=CD\\\\\sin55^o\approx0.8192\qquad(look\ at\ the\ picture)\\\\\text{Substitute:}\\\\0.8192=\dfrac{6\sqrt3}{CD}\\\\0.8192=\dfrac{10.3923}{CD}\qquad\text{multiply both sides by}\ CD\\\\0.8192CD=10.3923\qquad\text{divide both sides by 0.8192}\\\\CD\approx12.7[/tex]
