The Student Room Group

If X²+(3k+1)x+(4-9k) show that 9k²+42k-15

See attached:

IMG_0937.jpeg



Please don’t tell me this is another error?

I found b=30k.
(edited 1 month ago)
heya! repeated roots means b²-4ac is 0. your b value is (3k-1) a value is 1 and c value is (4-9k). sub these values into b²-4ac, expand and simplify, that should give you the ans
Original post by KingRich
See attached:
IMG_0937.jpeg
Please don’t tell me this is another error?
I found b=30k.


also k is not equal to b²-4ac
Well, there is in fact an error in the question, in that x^2+(3k+1)x+(4-9k) is not a quadratic equation. That said I suspect you probably aren't looking at this being the error.

The correct approach is as above.
Original post by KingRich
See attached:
IMG_0937.jpeg
Please don’t tell me this is another error?
I found b=30k.

The major error is that b^2 - 4ac =0 not k. also, you noted the expression for b wrong...
Reply 5
Original post by prathams07
heya! repeated roots means b²-4ac is 0. your b value is (3k-1) a value is 1 and c value is (4-9k). sub these values into b²-4ac, expand and simplify, that should give you the ans

I know what a repeated root means. I think you should re-read my original op. I already gave the solution. I wasn’t asking how to find it.

k is used to represent the integer. In this instance, I have to show k=0. So, because k represents the discrimination and so does the formula. Equating them means exactly the same.
Reply 6
Original post by WordsFiddle
The major error is that b^2 - 4ac =0 not k. also, you noted the expression for b wrong...

I noted b wrong? Oh, crap thank you
Reply 7
Original post by WordsFiddle
The major error is that b^2 - 4ac =0 not k. also, you noted the expression for b wrong...

also, two of you seem confused as to why I equate k=b^2-4ac.... How To Determine The Discriminant of a Quadratic Equation - YouTube... this video will explain it.
Original post by KingRich
also, two of you seem confused as to why I equate k=b^2-4ac.... How To Determine The Discriminant of a Quadratic Equation - YouTube... this video will explain it.

I'm not sure I follow why you think the discriminant = k when there the quadratic has repeated roots.

Remember the discriminant is exactly whatever goes inside the square root in the quadratic formula, and it precedes by a plus or minus sign, hence giving potentially 1 or 2 unique solutions. So if the discriminant is k, the roots are then "blah + sqrt(k)" and "blah - sqrt(k)", which might not be repeated...
(edited 1 month ago)
Original post by KingRich
also, two of you seem confused as to why I equate k=b^2-4ac.... How To Determine The Discriminant of a Quadratic Equation - YouTube... this video will explain it.

Oh so "k" given in the equation is not the same as what you equated b^2-4ac?

(The video just repeats what we already know about discriminants but thnx)
Reply 10
Original post by WordsFiddle
Oh so "k" given in the equation is not the same as what you equated b^2-4ac?
(The video just repeats what we already know about discriminants but thnx)

My error, I was meant to say D=b²-4ac.
Reply 11
Original post by tonyiptony
I'm not sure I follow why you think the discriminant = k when there the quadratic has repeated roots.
Remember the discriminant is exactly whatever goes inside the square root in the quadratic formula, and it precedes by a plus or minus sign, hence giving potentially 1 or 2 unique solutions. So if the discriminant is k, the roots are then "blah + sqrt(k)" and "blah - sqrt(k)", which might not be repeated...

I understand where the discriminant is and where it comes from within the quadratic formula. I also know if k=0 it’s a repeated root and k<0 has no roots and k>0 has two real roots..

I made the error of saying k but i was meant to say D=b²-4ac. It’s been a long day lol

Quick Reply

Latest