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a) all these points lie in the complex plane on the circle of the radius 5 with the center at the origin of coordinate system (z=0).
I will not plot them. Please do it yourself.
b) The geometric effect of multiplication by "i" is rotation in the angle (90 degrees) anti-clockwise.
c) The geometric meaning of = -1 is in that the multiplication of complex numbers by "i" two times is the same
as rotation of the complex plane in angle (180 degrees) anti-clockwise.
You may want to look into my lessons on complex numbers in this site
REVIEW of lessons on complex numbers.