A perfect square should be of the form:
(x + a)^2
So what must a be? Well, since we will multiply out brackets to make:
= x^2 + 2ax + a^2
Then we can see that 2a = -20 (from our original expression). So a = -10, therefore a^2 = 100.
So we need to add 100 to the expression :-)
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