1)x^2-4x+3=0;
D = 16 - 12 = 4 = 2^2;
x1 = 1; x2 = - 3.
2)x^2-6x+5=0;
D = 36 - 20 = 16= 4^2;
x1 = 1; x2 = 5.
3)x^2+8x-20=0;
D = 64 + 80 = 144 = 12^2;
x1 = - 10;
x2 = 2.
4)x^2+12x+32=0;
D = 144 - 128=16 = 4^2;
x1= - 8 ;
x2 = - 4.
6)x^2-4x-45=0;
D = 16 + 180 = 196 = 14^2;
x1 = 9;
x2 = - 5