Is there such an expression?

Question

I saw some notation on my friends notebook. What does that L shaped two lines mean? Goes below a1, a2 and a1-a2. I couldn't find anything related online. May this be about complex numbers? Can someone please explain? This is an electric circuit analysis notebook, by the way. Thanks in advance

Answer 1

I have seen this way of expressing complex numbers. The numerator on the left is magnitude $A$ at angle $\phi_1$, so in other notation would be $Ae^{i\phi_1}$ The equality would then be $\frac {Ae^{i\phi_1}}{Be^{i\phi_2}}=\frac ABe^{i(\phi_1-\phi_2)}$, which is true.