  back to trigonometry video lessons

# Same angle trigonometry function example

Question:

Given: sin a = 1/3, in which the angle a is in the range of (pi/2, pi). Find the value of cos (a - pi/6).

Solution:

The initial side of the angle alfa lies in the positive x-axis. The terminal side of the angle alfa lies in between pi/2 to pi, which is in Quadrant2. In Quadrant2, sin a > 0 and cos a < 0.

cos (a - pi/6)
= cos a cos pi/6 + sin a sin pi/6

We use formula: cos (a + b) = cos a cos b - sin a sin b. Note: cos pi/6 = cos 30o = sin 60o = Sqrt (3)/2. sin pi/6 = sin 30o = 1/2

cos (a - pi/6)
= [Sqrt (3)/2] cos a + (1/2) sin a

Now we are going to find cos a

sin2a + cos2a = 1
cos2a = 1 - sin2a

Because in Quadrant2, cos a < 0

cos a = - Sqrt (1 - sin2a)
given: sin a = 1/3
cos a = - Sqrt [1 - (1/3)2]
= - Sqrt (1 - 1/9)
= - Sqrt (8/9)
= - 2 Sqrt (2)/3
cos (a - pi/6)
= Sqrt (3)/2 × (- 2 Sqrt (2)/3) + (1/2)(1/3)
= 1/6 - Sqrt (6)/3
= 1/6 - 2/2 × Sqrt (6)/3
= 1/6 - 2 Sqrt (6)/6
= (1/6)(1 - 2 Sqrt (6))
= - 0.65

Therefore, the value of cos (a - pi/6) is - 0.65. Watch the video for more details.