Equation I simply don't understand I am working on some math problems from math books, and I have seen this in one math problem.
$x_1^2+x_2^2= (x_1+x_2)^2 - 2x_1x_2$
I simply don't understand this. Can you please explain this to me.  
The whole math problem is:
You have this equation
$x^2 - 3x + m^2 -1 = 0$
For what parameter m is:
$x_1^2 + x_2^2 > 3$

from Viete formulas
$$x_1+x_2=3$$
$$x_1\cdot x_2=m^2-1$$
$$x_1^2+x_2^2=(x_1+x_2)^2-2x_1x_2=3^2-2(m^2-1)>3$$
$$2m^2-2<6,m^2-4<0$$
$$(m-2)(m+2)<0$$
$$m\in(-2,2)$$
