Complex number question

Homework Statement

If z = cis @ where @ is acute, determine the modulus and argument of z-1

The Attempt at a Solution

As the moudlus of z is 1 z lies on the unit circle. And I can not think of anything more. I drew a graph to see how z-1 seems like in graph and stucked.

dx
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The modulus of x + iy is √(x² + y²) and the argument is tan-1(y/x). Draw the vector in the complex plane to see why.

That I know; but this thing is little different from the normal quetions where the number is multiplied to z such as -z, 2z etc which i can use that formula but here i think i should know some angles and shape of graph
I saw the answer and it was
moduls = 2 sin (@/2) argument = (@/2) + (pi/2)

dx
Homework Helper
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It's possible to do using just the formulas I wrote, but it's easier to do pictorially. Look at the attachment:

http://img99.imageshack.us/img99/86/picqrv.jpg [Broken]

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Wow i also drew some similar graph but couldnt understand what to do with that but by drawing the line at the middle everything become clear.
Thankyou so much!!!!!!!

dx
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No problem.