Given that one solution for the equation 12x2+kx=12\dfrac{1}{2x^{2} + kx} = \dfrac{1}{2} is x=−2x = -2, find

  1. (i)

    the value of k,

  2. (ii)

    a second possible value of x.

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Worked solution

3(i)

1. Substitute the given root into the equation and solve for $k$.

Since x=−2x=-2 is a solution,
12(−2)2+k(−2)=12\frac{1}{2(-2)^2+k(-2)}=\frac{1}{2}
2(−2)2−2k=22(-2)^2-2k=2
8−2k=28-2k=2
−2k=−6-2k=-6
k=3k=3

Answer

k=3k=3

3(ii)

1. Substitute $k=3$ into the original equation and solve for the other root.

Using k=3k=3,
12x2+3x=12\frac{1}{2x^2+3x}=\frac{1}{2}
Cross-multiply:
2=2x2+3x2=2x^2+3x
2x2+3x−2=02x^2+3x-2=0
Factorise:
(2x−1)(x+2)=0(2x-1)(x+2)=0
So x=12x=\dfrac{1}{2} or x=−2x=-2.
Since x=−2x=-2 is the given solution, the second possible value is
x=12.x=\dfrac{1}{2}.

Answer

x=−12x=-\dfrac{1}{2}

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