Question

Given alt="cot(\alpha )=-\sqrt{3}" align="absmiddle" class="latex-formula"> and $\pi/2 \ \textless \ \alpha \ \textless \ \pi$, find the exact values of the five remaining trigonometric functions:
1. $sin(\alpha )$
2. $cos(\alpha )$
3. $tan(\alpha )$
4. $csc(\alpha )$
5. $sec(\alpha )$

Answer 1

Immediately, by definition of cotangent, we find

tan(α) = 1/cot(α) = 1/(-√3)

⇒   tan(α) = -√3

Given that π/2 < α < π, we know that cos(α) < 0 and sin(α) > 0. In turn, sec(α) < 0 and csc(α) > 0.

Recall the Pythagorean identity,

cos²(α) + sin²(α) = 1

Multiplying both sides by 1/sin²(α) recovers another form of the identity,

cot²(α) + 1 = csc²(α)

Solving for csc(α) above yields

csc(α) = + √(cot²(α) + 1) = √((-√3)² + 1) = √4

⇒   csc(α) = 2

⇒   sin(α) = 1/2

Solve for cos(α) using the first form of the Pythagorean identity:

cos(α) = - √(1 - sin²(α)) = - √(1 - (1/2)²) = - √(3/4)

⇒   cos(α) = -√3/2

⇒   sec(α) = -2/√3