Trig values of 30 and 60 degrees

Geometry

Statement

The exact value of the trigonometry functions of 30° 30 \degree and 60° 60 \degree are:

sin(30°)=12 \sin(30 \degree) = \tfrac{1}{2}

cos(30°)=123 \cos(30 \degree) = \tfrac{1}{2}\sqrt{3}

tan(30°)=133 \tan(30 \degree) = \tfrac{1}{3}\sqrt{3}

sin(60°)=123 \sin(60 \degree) = \tfrac{1}{2}\sqrt{3}

cos(60°)=12 \cos(60 \degree) = \tfrac{1}{2}

tan(60°)=3 \tan(60 \degree) = \sqrt{3}

Proof

Construct triangle ABC ABC with A=B=C=60° \angle A = \angle B = \angle C = 60 \degree . Then draw altitude CD CD on AB AB so that C1=C2=30° \angle C_1 = \angle C_2 = 30 \degree , like the image below.

Let BD=m BD = m . Because this is an equilateral triangle, BC=2m BC = 2m .

Now find CD CD from the Pythagorean theorem:

CD=BC2BD2=(2m)2m2=4m2m2=3m2=m3 CD = \sqrt{BC^2 - BD^2} = \sqrt{(2m)^2 - m^2} = \sqrt{4m^2 - m^2} = \sqrt{3m^2} = m\sqrt{3}

Sine 30

From the triangle, note that  sin(C2)=BDBC \sin(\angle C_2) = \dfrac{BD}{BC} , so:

sin(30°)=m2m=12 \sin(30 \degree) = \frac{m}{2m} = \frac{1}{2}

Cosine 30

From the triangle, note that  cos(C2)=CDBC \cos(\angle C_2) = \dfrac{CD}{BC} , so:

cos(30°)=m32m=123 \cos(30 \degree) = \frac{m\sqrt{3}}{2m} = \tfrac{1}{2}\sqrt{3}

Tangent 30

From the triangle, note that  tan(C2)=BDCD \tan(\angle C_2) = \dfrac{BD}{CD} , so:

tan(30°)=mm3=13=133 \tan(30 \degree) = \frac{m}{m\sqrt{3}} = \frac{1}{\sqrt{3}} = \tfrac{1}{3}\sqrt{3}

Sine 60

From the triangle, note that  sin(B)=CDBC \sin(\angle B) = \dfrac{CD}{BC} , so:

sin(60°)=m32m=123 \sin(60 \degree) = \frac{m\sqrt{3}}{2m} = \tfrac{1}{2}\sqrt{3}

Cosine 60

From the triangle, note that  cos(B)=BDBC \cos(\angle B) = \dfrac{BD}{BC} , so:

cos(60°)=m2m=12 \cos(60 \degree) = \frac{m}{2m} = \tfrac{1}{2}

Tangent 60

From the triangle, note that  tan(B)=CDBD \tan(\angle B) = \dfrac{CD}{BD} , so:

tan(60°)=m3m=3 \tan(60 \degree) = \frac{m\sqrt{3}}{m} = \sqrt{3}


Proofs building upon this proof

Exact values of the trig functions

This table shows all exact values of sine, cosine and tangent.

Trig values of 15 degrees

This proof shows the exact value of sin(15), cos(15) and tan(15)