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(edited 6 years ago)
Original post by UCASLord
Find the range of the below equation, also indicating whether it is a one-to-one function or a one-to-many function.

f(x) = x^2 - 7x + 10, x R

I differentiated:

dy/dx = x - 7
x = 7
d^2y/dx^2 = 1

So it's a minimum.

But it's not actually a minimum. I noticed that the point where x = 7 is at y = 10, the same if x = 0. Now, if I take the point between these two coordinates, I get 3 1/2, which is the actual location of the minimum.

My question is why doesn't this differentiation give me the minimum point?


ddx(x2)=x\displaystyle \frac{\mathrm d}{\mathrm dx}(x^2) = x?
Reply 2
Original post by UCASLord
Find the range of the below equation, also indicating whether it is a one-to-one function or a one-to-many function.

f(x) = x^2 - 7x + 10, x R

I differentiated:

dy/dx = x - 7
x = 7
d^2y/dx^2 = 1

So it's a minimum.

But it's not actually a minimum. I noticed that the point where x = 7 is at y = 10, the same if x = 0. Now, if I take the point between these two coordinates, I get 3 1/2, which is the actual location of the minimum.

My question is why doesn't this differentiation give me the minimum point?


I guess what @_gcx is trying to point out is that ddx(x2)x \displaystyle \frac{d}{dx}(x^2) \neq x
Reply 3
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(edited 6 years ago)
Reply 4
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(edited 6 years ago)
Original post by UCASLord
Quick question though: is it a complete coincidence that I got the next point where y = 10, or is there a reason behind it?


Huh? What do you mean?

ddx(x27x+10)=2x7\frac{d}{dx}(x^2-7x+10)=2x-7 as other have pointed out. So setting this =0=0 gives x=72=3.5x=\frac{7}{2}=3.5 which is where the min point is. You get y=10y=10 at the y-intercept, which is nothing to do with it. It's a parabola so every other y coordinate point except the minimum will have two distinct x coordinates to go with it. In this case, x=0 and x=7.
(edited 6 years ago)

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