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

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

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?

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.

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(7) Jump to unreadPost a commentwait hold on ill get you the answer

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

standard form is ax2 + bx + c=0

:thumbsup:

Also, don't forget that if K is variable you can have quite a few different roots for the equation.

However, we still think there are to many unknowns to be able to solve this?

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

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