Question 61672
Why do we call the polar angle of complex number an argument???
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Go to this site:
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http://members.aol.com/jeff570/a.html
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and scroll down to "argument". 
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Why did they call the magnitude, or the length of the line or vector from the origin to the point (x,y) the "Modulus" of the complex number x + yi?  The Mathematician Jean-Robert Argand (1768–1822) first use the term "modulus".
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My answer to both:  They had to call them something! LOL. I sort of like "argument" and "modulus" because both the word AnGle and the word "ArGument" both have an "A" first and a "G" third.  You can remember "Modulus" because "Magnitude" also starts with an M.
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Edwin