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# C1 Completing the square watch

1. This feels so simple and I don't know why I can't do it.

Express 4x^2 -32x +17 in the form a(x-b)^2 +c

So I have done
4(x-8x)+17
then 4(x-4)^2 -16 +17 (+1)
I'm not sure if 16 should be positive or negative but I don't see how it works either way because it doesn't go back to what it was when I expand it. What am I doing wrong?
This feels so simple and I don't know why I can't do it.

Express 4x^2 -32x +17 in the form a(x-b)^2 +c

So I have done
4(x-8x)+17
then 4(x-4)^2 -16 +17 (+1)
I'm not sure if 16 should be positive or negative but I don't see how it works either way because it doesn't go back to what it was when I expand it. What am I doing wrong?
You haven't multiplied the -16 by its 4.

.

ETA: Typo: the last shouldn't be there. That's what happens when I don't reread what I've written.
3. (Original post by Smaug123)
You haven't multiplied the -16 by its 4.

.
Where did
+4*16+17 come from?
Where did
+4*16+17 come from?
Apologies, typo - I didn't reread. Scratch the 4*16.

5. (Original post by Smaug123)
Apologies, typo - I didn't reread. Scratch the 4*16.

Ahh okay, so it equals 4(x-4)^2 -47?
Then if that's right and you were to solve it when it =0 so that the answer is in surd form would you just start off by going 4(x-4)^2 = 47 and then take everything to the other side?
Ahh okay, so it equals 4(x-4)^2 -47?
Then if that's right and you were to solve it when it =0 so that the answer is in surd form would you just start off by going 4(x-4)^2 = 47 and then take everything to the other side?
That's right. So to solve the equation, you'd do , so .
7. (Original post by Smaug123)
That's right. So to solve the equation, you'd do , so .
Thank you!

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