Posted by **Cambridge** on Thursday, January 2, 2014 at 11:24am.

Prove algebraically that |w+z| is less than or equal to |w|+|z| for any complex numbers w and z, where || is the magnitude.

After letting w = a+bi and z = c + di and doing some plugging in etc. I got that

ac-bd <= b^2 + sqrt (a^2+b^2)(c^2+d^2) + d^2

But is my proof complete? I know logically sqrt (a^2+b^2)(c^2+d^2) is a bit less than ac+ad+bc+bd but now what?

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