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# Can anyone help with this Maths problem?

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We've all been trying and can't work it out. Find the values of k for which k(x to the power 2) + 8x +5 =0 has real roots. the answer is: x is smaller than zero, x is greater than 1. Bu… Read More
8y, 9m agoPosted 8 years, 9 months ago
We've all been trying and can't work it out.

Find the values of k for which k(x to the power 2) + 8x +5 =0 has real roots.

the answer is: x is smaller than zero, x is greater than 1.

But despite various attempts and graphs drawn we cannot get from the problem to the answer
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8y, 9m agoPosted 8 years, 9 months ago
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(7)
1 Like #1
oh maths, i like maths :D
wait hold on ill get you the answer
1 Like #2
42 - Always the answer.....
#3
the answer is not "answering the question"
why is the answer talking about x when we wanna know about k

To find the real roots, it comes to the formulae -b+-(sqrt)"bsquared - 4ac" etc
using only bsquared -4ac, it is possible to determine if there are real roots or no roots
bsquared-4ac = 64-20k
64-20k must be =>0 if to have real roots
so 64 - 20k => 0
solving makes 3.2 => k
or k must be equal or less than 3.2 if the equation was to have real roots

Edit: it says "real roots", so I guess one root is not good enough, so make the answer just less than 3.2
1 Like #4
Use the Quadratic formula -b + or - the square root of b squared - 4ac divided by 2a

standard form is ax2 + bx + c=0
:thumbsup:
1 Like #5
Its a 2 degree polynomial or more commonly known as a quadratic equation. The graph should be a parabola but this link will give you the real roots http://www.ajdesigner.com/phpquadraticequation/quadraticequation.php

Also, don't forget that if K is variable you can have quite a few different roots for the equation.
#6
Thanks for your help so far. We've now worked out that the answer is wrong - it seems the publishers of the textbook have actually missed that answer out and wrongly numbered the answers.
However, we still think there are to many unknowns to be able to solve this?
1 Like #7
Artonox
the answer is not "answering the question"
why is the answer talking about x when we wanna know about k

To find the real roots, it comes to the formulae -b+-(sqrt)"bsquared - 4ac" etc
using only bsquared -4ac, it is possible to determine if there are real roots or no roots
bsquared-4ac = 64-20k
64-20k must be =>0 if to have real roots
so 64 - 20k => 0
solving makes 3.2 => k
or k must be equal or less than 3.2 if the equation was to have real roots

Edit: it says "real roots", so I guess one root is not good enough, so make the answer just less than 3.2

kensington143
Thanks for your help so far. We've now worked out that the answer is wrong - it seems the publishers of the textbook have actually missed that answer out and wrongly numbered the answers.
However, we still think there are to many unknowns to be able to solve this?

I don't think it is asking you to solve, just what values for K would give real roots, hence not a negative sqauare root. Look again at this first quote, just consider the sqaure root part of your quadratic formula, if this is zero or greater your roots are real.

You should only be considering the discriminant