Hi can you help me with:

Suppose z = cis A where A is acute. Find the argument of z + 1:

I understand how to draw the vectors and all on the Argand diagram, so the question with the answers and all are in the first image.

However,t o find the argument of z+1, I don't understand, my understanding is that the argument of z+1 is the red angle, as shown in the next picture. Why is it wrong? For the answer is that the argument of z+1 is pi/2 since it is a rhombus, but I don't see how, not from my understanding of what a z+1 argument is!

Please help, thank you so much!

Thank you!