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