How can we compute dx in terms of R and θ in this given right triangle?

[quantum13]

Homework Statement

This is part of a find the electric field problem. I've narrowed down the part I find confusing.

Consider a right triangle, with legs of length x and R. Angle θ is opposite x. Leg x is a segment of a ray starting where lines x and R intersect. There is a differential length dx along line x (it is at a vertex of the right triangle). Find dx in terms of R and θ.

Homework Equations

standard trig functions, pythagorean theorem?

The Attempt at a Solution

The answer according to the solutions manual is dx = (R/cos^2 θ) dθ. Obviously, I cannot understand at all where that came from.

I did get R tan ( θ + dθ) = x + dx but that isn't very helpful either

 

[Redbelly98]

It sounds like R is fixed, while x and θ can vary.

Can you write an expression for x in terms of R and θ?