Is this statement acceptable?

[Clockclocle]

Homework Statement

Is this statement acceptable?

Relevant Equations

..........

I understand the approximation statement but he divide the |delta t| in the left but only delta t on the right. Is it true because delta phi would have the same sign as delta t ?

 

[kuruman]

Yes. By definition $\Delta \phi$ is positive and the angle increases in the direction of the arrow in the top figure. $\Delta t$ is always positive.