book Maths Puzzle Numerical Ability

simplify 4x-5(x-7) a -x + 35 b) -x-35 c) -x-7 d) 9x+35

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book Other Question

Hello...

I am having some trouble with a proof. I am to use:

(a + bi)(a - bi)

The problem asks:

Show that for a + bi not equal to 0 this product is a positive real number.

Thanks for your help.
-1 + 5i
________
3 + 2i