question:
if , prove that the quadratic expression is positive for all real values of x when .
my answer:
let
if f(x) >0 for all real values of x, it has no real roots
wil produce a negative number let call it g = d
since
then
when
, f(x) > 0 for all real values of x
Please I want to know if my proof correctly answers the question please tell where i went wrong
thank you
quadratic equation question
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Online3ReputationRep:(Original post by bigmansouf)
question:
if , prove that the quadratic expression is positive for all real values of x when .
my answer:
let
if f(x) >0 for all real values of x, it has no real roots
wil produce a negative number let call it g = d
since
then
f(x) > 0 for all real values of x
Please I want to know if my proof correctly answers the question please tell where i went wrong
thank you
You are asked to prove that if then . What you have attempted to do instead is prove that if then and then use this to prove .
But you don't need the first part since you're told already that . The major problem is that knowhere in your working have you actually proved why implies that .
You could talk about the discriminant/no real roots to show this but I assume from your working that you need to prove it by completing the square? You correctly completed the squre but didn't use it for anything.
Think about why this
must always be positive.
One more thing:
This doesn't mean much. I assume you're trying to say that is a complex number but the set of complex numbers includes the reals and the imaginary numbers. What you mean to say is that is imaginary. There is a mathematical way to write this but you may as well just write it in words and I recommend this unless you're confident with using the formal notation. 
 Thread Starter
Offline3ReputationRep:(Original post by notnek)
Firstly, you should have more working when you complete the square.
You are asked to prove that if then . What you have attempted to do instead is prove that if then and then use this to prove .
But you don't need the first part since you're told already that . The major problem is that knowhere in your working have you actually proved why implies that .
You could talk about the discriminant/no real roots to show this but I assume from your working that you need to prove it by completing the square? You correctly completed the squre but didn't use it for anything.
Think about why this
must always be positive.
One more thing:
This doesn't mean much. I assume you're trying to say that is a complex number but the set of complex numbers includes the reals and the imaginary numbers. What you mean to say is that is imaginary. There is a mathematical way to write this but you may as well just write it in words and I recommend this unless you're confident with using the formal notation.
here goes my second attempt
let
since \rightarrow
since , has a least value of when
since
if is the least values, taking the input values of x that are symmetrical about i.e gives
since the least value
any inputs of x that are symmetrical will also give output of the same where
( f(x) for points symmetrical about x = b/2a will be > 0 and > (4acb^2) / (4a)
explaining incase you dont understand
when ,
Please i understand you said to stick to using words i want to do better and writing mathematical notation was advised by my teacher . if i am wrong with the notation please show me the correct way. I know that the last point which i was saying ( f(x) for points symmetrical about x = b/2a will be > 0 and > (4acb^2) / (4a)
I want to improve my mathematical notation as i am very slow in writing
thank you 
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You're over complicating it. From
you know that
(i) the squared part is always greater than or equal to 0, because that's a basic property of a square.
(ii) you're multiplying the square by , which you know is . So .
Now, it would be really nice if we could prove that was positive, in which case, we would know that was a sum of two positive terms and hence would be positive itself.
But, how to do that... well, for a fraction to be positive, we want both its numerator and denominator to be positive. You can see trivially that the denominator is always positive here (because you're given that, so must also be ).
You're reduced the entire problem to proving that the numerator is also positive. So, how can we prove that ? Oh... what a coincidence! This is precisely what we were assuming!
In writing the actual proof up, it would go something like this:
, since the first term is nonnegative. Since we can then conclude that and so also. So is the sum of two positive terms and is hence positive itself.
NB: When I say "two positive terms" I really mean "sum of one nonnegative term and positive term", but the latter is a bit of a mouthful.... and the distinction hardly matters there anyway, I've explicitly used nonnegative where the distinction does matter. 
 Thread Starter
Offline3ReputationRep:(Original post by Zacken)
I'm going to try explaining this to you because I don't feel like subtle hints are working very welly. (sorry for butting in, notnek!)
You're over complicating it. From
you know that
(i) the squared part is always greater than or equal to 0, because that's a basic property of a square.
(ii) you're multiplying the square by , which you know is . So .
Now, it would be really nice if we could prove that was also positive, in which case, we would know that was a sum of two positive terms and hence would be positive itself.
But, how to do that... well, for a fraction to be positive, we want both its numerator and denominator to be positive. You can see trivially that the denominator is always positive here (because you're given that, so must also be ).
You're reduced the entire problem to proving that the numerator is also positive. So, how can we prove that ? Oh... what a coincidence! This is precisely what we were assuming!
In writing the actual proof up, it would go something like this:
, since the first term is nonnegative. Since we can then conclude that and so also. So is the sum of two positive terms and is hence positive itself.
can i conclude from what you said that if a quadratic function is a sum of two squares therefore f(x) > 0 is positive for all real values of x and thus will have no real roots
I am just trying to make note on this question 
Offline3ReputationRep:
Once you complete the square, you're left with
From here you know that: (ALWAYS due to squaring)
One of the conditions you are given is therefore
The other condition is therefore
Summing the two terms greater than 0 will obviously give you something greater than 0 therefore
Why are you talking about the roots???
EDIT: Nevermind, Zacken beat me to it. 
Offline3ReputationRep:(Original post by bigmansouf)
thank you woow i dont know what to say but thanks
can i conclude from what you said that if a quadratic function is a sum of two squares therefore f(x) > 0 is positive for all real values of x and thus will have no real roots
I am just trying to make note on this question
In another question, where you could have then you can't quite say that , the best you can say is that , because , not .
However, if you had something like then you can definitely say that and so it has no real roots, since both brackets can't be simultaneously 0 for distinct .
In fact, you use this technique a lot when you're asked to do stuff like "prove that has no real roots". You do: so that means that has no real roots. 
Offline3ReputationRep:(Original post by RDKGames)
... 
 Thread Starter
Offline3ReputationRep:(Original post by Zacken)
Notnek deserves it more than I do.
Indeed! (although this is not the case in this particular question)  here it's "if a function is a sum of two positive things for all real values of x, then the function is positive for all real values of x".
In another question, where you could have then you can't quite say that , the best you can say is that , because , not .
However, if you had something like then you can definitely say that and so it has no real roots.
In fact, you use this technique a lot when you're asked to do stuff like "prove that has no real roots". You do: so that means that has no real roots.
i am going to work harder i feel i got played around by bostock and chandler (making easy stuff look difficult) 
Offline3ReputationRep:(Original post by bigmansouf)
thank i gave him a rep so did u and RDK games
i am going to work harder i feel i got played around by bostock and chandler (making easy stuff look difficult)
That's a good idea! You should probably start off with easier resources and then use B&C to supplement your learning instead of learning solely from it  some of the stuff is a bit archaic.
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