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Question

Find the value of k for which the quadratic equation (3k+1)x2+2(k+1)x+1=0 has equal roots. Also, find the roots.

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Solution

For the equation to be a perfect square, the determinant should be zero.
Hence,
b squared minus 4 a c equals 0h e r e comma space a equals 3 k plus 1 comma space b equals 2 left parenthesis k plus 1 right parenthesis space c equals 1left parenthesis 2 left parenthesis k plus 1 right parenthesis right parenthesis squared minus 4 left parenthesis 3 k plus 1 right parenthesis equals 04 left parenthesis k squared plus 2 k plus 1 right parenthesis minus 4 left parenthesis 3 k plus 1 right parenthesis equals 0k squared plus 2 k plus 1 minus 3 k minus 1 equals 0k squared minus k equals 0k left parenthesis k minus 1 right parenthesis equals 0k equals 0 space o r space 1
Now if k=0 then the equation becomes,
x squared plus 2 x plus 1 equals 0 x squared plus x plus x plus 1 equals 0 x left parenthesis x plus 1 right parenthesis plus left parenthesis x plus 1 right parenthesis equals 0 left parenthesis x plus 1 right parenthesis left parenthesis x plus 1 right parenthesis equals 0 x equals negative 1 comma negative 1
Therefore when k=0 then the roots of the equation are -1 and -1.

Now when k=1 then the equation becomes,
4 x squared plus 4 x plus 1 equals 0 4 x squared plus 2 x plus 2 x plus 1 equals 0 2 x left parenthesis 2 x plus 1 right parenthesis plus left parenthesis 2 x plus 1 right parenthesis equals 0 left parenthesis 2 x plus 1 right parenthesis left parenthesis 2 x plus 1 right parenthesis equals 0 x equals negative 1 half space comma space minus 1 half
Therefpore when k=1 then the roots of the equation are negative 1 halfand negative 1 half


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