Question

Triangle ΔABC has side lengths of a = 18, qrt{3}" alt="b=18\sqrt{3}" align="absmiddle" class="latex-formula"> and c = 36 inches.
Part A: Determine the measure of angle A

Part B: Show how to use the unit circle to find tan A

Part C: Calculate the area of ΔABC.

Answer 1

The measure of angle A is 30 degree, the value of tan A is 1/√3 and the area of triangle ABC is 280.6 squared inches.

What is the law of cosine?

When the three sides of a triangle is known, then to find any angle, the law of cosine is used.

It can be given as,

$\angle A=\cos^{-1}\left(\dfrac{b^2+c^2-a^2}{2bc}\right) \\\angle B=\cos^{-1}\left(\dfrac{a^2+c^2-b^2}{2ac}\right) \\\angle C=\cos^{-1}\left(\dfrac{a^2+b^2-c^2}{2ab}\right)$

Here, a,b and c are the sides of the triangle and A,B and C are the angles of the triangle.

Triangle ΔABC has side lengths of a = 18, b=18√3  and c = 36 inches.

• Part A: Determine the measure of angle A

Put the value, in the cosine law, the measure of angle A.

$\angle A=\cos^{-1}\left(\dfrac{b^2+c^2-a^2}{2bc}\right) \\\angle A=\cos^{-1}\left(\dfrac{(18\sqrt{3})^2+36^2-18^2}{2(18\sqrt{3})(36)}\right) \\\angle A=0.5236\rm\; rad\\\angle A=30^o\rm\; degree$

• Part B: Show how to use the unit circle to find tan A

Using the chart of unit circle, the value of tangent A can be found out. The tangent A is,

$\tan A=\tan 30^o\\\tan A=\dfrac{1}{\sqrt{3}}$

• Part C: Calculate the area of ΔABC.

Use the following formula to find area of ΔABC.,

$A=\dfrac{ab.\sin C}{2}\\A=\dfrac{18\times18\sqrt{3}.\sin C}{2}\\A=280.6\rm\; in^2$

Thus, the measure of angle A is 30 degree, the value of tan A is 1/√3 and the area of triangle ABC is 280.6 squared inches.

Learn more about the law of cosine here;

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