[tex]y=ax^2+bx+c\to y=x^2-6x+5[/tex]
a = 1 > 0 therefore, this function has minimum.
Use [tex](*)\ (a-b)^2=a^2-2ab+b^2[/tex]
[tex] x^2-6x+5=x^2-2\cdot x\cdot3+5=\underbrace{x^2-2\cdot x\cdot3+3^2}_{(*)}-3^2+5\\\\=(x-3)^2-9+5=(x-3)^2-4 [/tex]
Answer: C) minimum value at −4.
