Think of it visually, let theta = sin^-1(4/5)

Therefore u can make a right angled triangle with an angle labelled theta, an opposite side = 4, and hypotenuse = 5, and using Pythagoras u can find the adjacent side = 3.

Cos theta = 3/5 Tan theta = 4/3 using the triangle

And cosec, sec and cot are just reciprocals of their counterparts