In Fig. 5.47, the value of cos ϕ is
As we know that sum of the angles of the straight line is 180°,
Therefore,
∠ θ + ∠90 + ∠ϕ = 180
∠ θ + ∠ϕ = 90
In ∆ABC,
sin θ = 4/5 = p/h
Putting θ = 90 - ϕ
We get,
sin (90 - ϕ) = 4/5
As,
Sin (90 - ϕ) = cos ϕ
cos ϕ = 4/5