The Student Room Group

Is it possible to find the max/min point of a quadratic with no real roots?

e.g. y = x2 - 4x + 5
(edited 7 years ago)
Reply 1
Original post by honour
e.g. y = x2 - 4x + 5


Complete the square so it is in the form (x+a)2+b(x+a)^2 +b and then the minimum/maximum point will be at (a,b)(-a,b)
Reply 2
Original post by honour
e.g. y = x2 - 4x + 5


Why not?
Yes - the roots just dictate the x intercepts, but the the shape of the graph will still remain intact thus the max/min points can be identified.
Reply 4
Original post by solC
Complete the square so it is in the form (x+a)2+b(x+a)^2 +b and then the minimum/maximum point will be at (a,b)(-a,b)


Thanks, I previously thought this was only possible for quadratics with real roots.
Reply 5
You could also find the derivative and equate it to 0 and solve for x. This will give you the x value of the min/max point. You can find y by calculating f(x)
Original post by agrajaag
You could also find the derivative and equate it to 0 and solve for x. This will give you the x value of the min/max point. You can find y by calculating f(x)


I think differentiation might be a bit too much for him at this point xD
Reply 7
Perhaps the fact that you're asking this question suggests that you don't really understand what minimum/maximum points of a quadratic curve are. They are the turning points of the curve (in the case of quadratics) and every quadratic curve has a turning point since if y=ax2+bx+c y=ax^2+bx+c then finding the turning points we solve dy/dx=0 dy/dx=0 so 2ax+b=0 2ax+b=0 and so there is always a (real) solution. You could also complete the square to get the same result.

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