Roots

Algebra Level 2

Determine the sum of all values of k k for which the following equation in x x has 2 equal roots:

k x 2 + 2 x 2 3 k x + k = 0 kx^2+2x^2-3kx+k=0


The answer is 1.6.

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1 solution

k x 2 + 2 x 2 3 k x + k = 0 kx^2+2x^2-3kx+k=0

( k + 2 ) x 2 3 k x + k = 0 (k+2)x^2-3kx+k=0 \color{#3D99F6}\large \implies It is a quadratic equation in the form of a x 2 + b x + c = 0 ax^2+bx+c=0

So, a = k + 2 , b = 3 k a=k+2, b=-3k and c = k c=k

So that the roots are equal, the discriminant must be equal to 0 0 . That is, b 2 4 a c = 0 b^2-4ac=0

So, we have

( 3 k ) 2 4 ( k + 2 ) ( k ) = 0 (-3k)^2-4(k+2)(k)=0 \color{#3D99F6}\large \implies 9 k 2 4 k ( k + 2 ) = 0 9k^2-4k(k+2)=0 \color{#3D99F6}\large \implies 9 k 2 4 k 2 8 k = 0 9k^2-4k^2-8k=0 \color{#3D99F6}\large \implies 5 k 2 8 k = 0 5k^2-8k=0 \color{#3D99F6}\large \implies k ( 5 k 8 ) = 0 k(5k-8)=0

k = 0 k=0

5 k 8 = 0 5k-8=0 \color{#3D99F6}\large \implies 5 k = 8 5k=8 \color{#3D99F6}\large \implies k = 8 5 k=\dfrac{8}{5}

Finally,

0 + 8 5 = 0+\dfrac{8}{5}= 1.6 \color{#D61F06}\boxed{\large 1.6}

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