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TalkOrigin
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Hi, I've been attempting to teach myself mathematics in preparation for further study and I'm currently going through quadratic equations. In case it helps, I'm studying to go back to university, I'll be doing a physics degree with a foundation year. I'm just trying to learn as much math as possible before I start, and I'm working through the UK A-Level Math syllabus, OCR, I'm on C1 at the moment. I seem to be fine with quadratic equations except for when the equation is in the form ##c + bx - a x^2## instead of ##a x^2 + bx + c##
I know if it's ##c + bx + a x^2## I can just flip everything round so it's in the standard form and it doesn't give me too much trouble, but when the coefficient of ##x^2## is a negative number, I just don't know how to approach the problem. Here is one such problem I'm stuck on:
"Use the completed square form to find as appropriate the least or greatest value of each of the following expressions, and the value of x for which this occurs".
##4 + 6x - x^2##
I'm sure it's simple, I just don't know how to do it. I've tried looking online but I can't find anything addressing how to solve a question like this specifically. I'm probably just being an idiot and missing some incredibly obvious step. My attempt at solving it was:
##4 + 6x - x^2##
##4 + (3-x)^2 - 9##
then, do ##4 + -9 = -5## So
##(3-x)^2 - 5##
Least Value - I got -5, although apparently the answer is 13. I know I can't be a million miles off as I don't think it's coincidence that 4+9=13, I just can't see where I'm going wrong. I got the value of x for when this least value occurs correct somehow, at 3.
Anyway thanks for the help, I don't know why I get so confused when the equations are formulated in this way. I don't even need someone to tell me what the answer is, but how to solve an equation when it's in that form would be great, maybe to point out where I'm going wrong also.
Thanks again
I know if it's ##c + bx + a x^2## I can just flip everything round so it's in the standard form and it doesn't give me too much trouble, but when the coefficient of ##x^2## is a negative number, I just don't know how to approach the problem. Here is one such problem I'm stuck on:
"Use the completed square form to find as appropriate the least or greatest value of each of the following expressions, and the value of x for which this occurs".
##4 + 6x - x^2##
I'm sure it's simple, I just don't know how to do it. I've tried looking online but I can't find anything addressing how to solve a question like this specifically. I'm probably just being an idiot and missing some incredibly obvious step. My attempt at solving it was:
##4 + 6x - x^2##
##4 + (3-x)^2 - 9##
then, do ##4 + -9 = -5## So
##(3-x)^2 - 5##
Least Value - I got -5, although apparently the answer is 13. I know I can't be a million miles off as I don't think it's coincidence that 4+9=13, I just can't see where I'm going wrong. I got the value of x for when this least value occurs correct somehow, at 3.
Anyway thanks for the help, I don't know why I get so confused when the equations are formulated in this way. I don't even need someone to tell me what the answer is, but how to solve an equation when it's in that form would be great, maybe to point out where I'm going wrong also.
Thanks again
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