Whenever you have the square root of a negative value, you may change the sign under the radical and place an i outside the radical as a factor in its place.
Another way to look at it is:
Since we may write
my edexcel further maths book says i need to solve this equation giving the answer in terms of the positive number a.
x^2 + a^2 = 0.
the answer in the back of the text book is x = + or - ai
but i dont see how i can get this answer
could someone please explain
Yes, this is a complex number problem where i refers to iota, a imaginary number which is equal to √ -1. it is introduce to solve this type of problems where √ (negative number) occur.