Answer:
See Below.
Step-by-step explanation:
Since ΔABC is an equilateral triangle, this means that ∠A, ∠B, and ∠C all measure 60°.
Furthermore, all sides of the triangle measure 2a.
We know that AD⊥BC. Since this is an equilateral triangle, any altitude will be a perpendicular bisector. Therefore, BD = DC, ∠CAD = 30° and ∠C = 60°.
Additionally, the measures of the segments are: BD = DC = (2a) / 2 = a.
Let’s use the right triangle on the right. Here, we have that DC = a and AC = 2a.
Recall that sine is the ratio of the opposite side to the hypotenuse. Therefore, sin(C) or sin(60°) will be AD / AC. We can find AD using the Pythagorean Theorem:
[tex]a^2+b^2=c^2[/tex]
a is DC, b is AD, and c is AC.
Substitute in appropriate values:
[tex]a^2+(AD)^2=(2a)^2[/tex]
Solve for AD:
[tex]\displaystyle \begin{aligned} a^2+(AD)^2&=4a^2\\(AD)^2&=3a^2\\AD&=\sqrt{3a^2}=a\sqrt{3}\end{aligned}[/tex]
Sine is the ratio of the opposite to the hypotenuse:
[tex]\displaystyle \sin(C)=\sin(60\textdegree)=\frac{\text{opposite}}{\text{hypotenuse}}[/tex]
Substitute:
[tex]\displaystyle \sin(60\textdegree)=\frac{AD}{AC} = \frac{a\sqrt{3}}{2a}[/tex]
Simplify. Hence, regardless of the value of a:
[tex]\displaystyle \sin(60^\circ)=\frac{\sqrt3}{2}[/tex]
