How can I find "dx" in a straight line of electric charge?(Picture)

[MatinSAR]

Homework Statement

How can I find "dx" in a straight line of electric charge?(Details in Picture)

Relevant Equations

r=R/cos(teta)
x=rtan(teta)

This is the question ... I have it's solution ...

My problem : I can't understand why dx=R/cos^2(teta) dteta
I have thought many hours but I couldn't find it's reason ... Can anyone please help with this ?!

 

[ergospherical]

If $x = R\tan{\theta}$, then $dx/d\theta= \dots$

 

[MatinSAR]

If $x = R\tan{\theta}$, then $dx/d\theta= \dots$

I got it ... I have learned this in a wrong way but now I know what was wrong ... Thanks a lot