Roots of a Quadratic

Algebra Level 1

The equation 2 x 2 62 x + k = 0 2x^2 - 62x + k = 0 has two real roots, one of which is 1 more than twice the other. Find the value of k k .


The answer is 420.

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7 solutions

Kunal Das
Nov 18, 2013

Let the roots be r and 2r+1 ( x r ) ( x 2 r 1 ) = 0 (x-r)(x-2r-1)=0 = > x 2 ( 3 m + 1 ) x + 2 m 2 + m = 0 => x^{2}-(3m+1)x+2m^{2}+m=0 From the equation, comparing coefficients of x we get = > 3 m + 1 = 62 / 2 = 31 => 3m+1=62/2=31 i.e. m = 10 m=10 and comparing coefficients of last term we get k / 2 = 2 m 2 + m = 210 = > k = 420 k/2=2m^{2}+m=210 => k=420

correct

Abdussamad Bello - 7 years, 6 months ago
Waqar Zaman
Nov 18, 2013

let the first root be "t" then by given condition the second root is "2t+1" now, sum of roots= t+2t+1 = -b/a = 62/2 so that t=10 product of roots t*(2t+1)= c/a = k/2 by putting value of t , k=420

let roots are x, 2x+1. 31=x+2x+1 x=10 K/2=10*21 k=420

I started by trying to simplify the equation. Dividing both sides by two, the equation becomes:

x 2 31 x + k 2 = 0 x^2 -31x + \frac k2 = 0

By simple factoring (given that the answers are both real integers), we come up with the equation:

( x [ k + 1 ] ) ( x k 2 ) = 0 (x - [k+1])(x - \frac {k}{2}) = 0

Using the FOIL method, we can then presume that:

k 2 x + ( k + 1 ) x = 31 x \frac {-k}{2}x + -(k+1)x = -31x

This gives us:

k x 2 k x x = 31 x \frac {kx}{2} - kx - x = -31x

Factoring x out of both sides, we get:

k 2 + k + 1 = 31 \frac {k}{2} + k + 1 = 31

3 k 2 = 30 \frac {3k}{2} = 30 , and we get k = 20.

If k = 20, then we can presume that one of the roots is 10. The other root would then be 21, fulfilling the problem requirements.

21 x 10 = 210, but because we divided the equation at the very beginning by 2, the value of k would be:

21 x 10 x 2 = 420 21 x 10 x 2 = \boxed {420}

Michael David Sy - 7 years, 6 months ago

how do you solve this????

Ryan NOOB - 7 years, 6 months ago

Can you use words to explain what you mean?
Can you state the techniques that you are using?

Calvin Lin Staff - 7 years, 6 months ago
Abubakarr Yillah
Jan 6, 2014

Let one of the roots be α \alpha

and let the other root be 2 α + 1 {2}\alpha+{1}

from the equation given, a = 2 {a}={2} b = 62 {b}={-62}

and c = k {c}={k}

sum of the roots equals α + 2 α + 1 = b a = 62 2 \alpha+{2}\alpha+{1}=\frac{-b}{a}=\frac{62}{2}

3 α + 1 = 31 {3}\alpha+{1}={31}

α = 10 \alpha={10}

product of the roots equals α ( 2 α + 1 ) = c a = k 2 \alpha({2}\alpha+{1})=\frac{c}{a}=\frac{k}{2}

2 ( α ) 2 + α = k 2 {2}(\alpha)^2+\alpha=\frac{k}{2}

k = 4 ( α ) 2 + 2 α {k}={4}(\alpha)^2+{2}\alpha

but α = 10 \alpha={10}

thus, k = 4 ( 10 ) 2 + 2 ( 10 ) {k}={4}({10})^2+{2}({10})

k = 400 + 20 {k}={400}+{20}

Hence k = 420 {k}=\boxed{420}

Luan Moitinho
Nov 19, 2013

x1+x2= -b/a

x1 *x2= c/a

x1+2*(x2) +1 = -(-62)/2>>>>x1=10

x2=2*x2 +1>>>>> X2=21

10 *21= K/2

K= 420

Lutful Kabir Rumi
Nov 18, 2013

two real roots m,n m+n= - (-62)/2 =31, m=2n+1, 2n+1+n=31, n=10, m=21,

product of two roots =m n =21 10=210=k/2, k=420.

Did you use Vietta's Theorem?

Hiren Saravana - 7 years, 6 months ago

what lesson is this ????

Amro Faidalla - 7 years, 6 months ago
Andre Yudhistika
Jan 5, 2014

the equation has 2 real roots which are x1 and x2 a=2 b=-62 c=k

x2=2x1+1

x1+x2=x1+2x1+1=-b/a=62/2=31

x1=10

x1 x2=x1 (2x1+1)=210=c/a=k/2

therefore k=210*2 =420

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