SOLUTION: in a quadratic equation, a student made a mistake in copying the coefficient of x^(2) and got the roots of 2 and 3. Another student made a mistake copying the constant term and got

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Question 1104988: in a quadratic equation, a student made a mistake in copying the coefficient of x^(2) and got the roots of 2 and 3. Another student made a mistake copying the constant term and got the roots of 4 and 6. What are the correct roots?

Answer by ikleyn(52777) About Me  (Show Source):
You can put this solution on YOUR website!
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0.  Let the original equation be  ax%5E2+%2B+bx+%2B+c = 0.     (1)


1.  After the 1-st student incorrectly copied the coefficient at  x%5E2,  the equation took the form

    dx%5E2+%2B+bx+%2B+c = 0.     (2)


    Since its roots are 2 and 3, we have this decomposition

    dx%5E2+%2B+bx+%2B+c = d*(x-2)*(x-3),     or

    dx%5E2+%2B+bx+%2B+c = dx%5E2+-+5dx+%2B+6d.

    So, the original equation (1) has the two lowest degree terms -5dx + 6d:

        ax%5E2+%2B+bx+%2B+c = ax%5E2+-+5dx+%2B+6d    (3)

    with some unknown coefficients  "a"  and  "d".



2.   After the 2-nd student incorrectly copied the constant term,  the equation took the form

    ax%5E2+-+5dx+%2B+e = 0.     (4)


    Since its roots are 4 and 6, we have this decomposition

    ax%5E2+-+5dx+%2B+e = a*(x-4)*(x-6),     or

    ax%5E2+-+5dx+%2B+e = ax%5E2+-+10ax+%2B24a.

    It implies  -5d = -10a,  which in turn  implies  d = 2a.

    Now from (3) we conclude that the original equation (polynomial) is/was

        ax%5E2+-+5dx+%2B+6d = ax%5E2+-+10ax+%2B+12a.

    Its roots are the same as for equation  x%5E2+-+10x+%2B+12 = 0.

    And they are  x%5B1%2C2%5D = %2810+%2B-+sqrt+%2810%5E2+-4%2A12%29%29%2F2 = 5+%2B-+sqrt%2813%29.


Answer.  The roots of the original equation are  x%5B1%5D = 5+%2B+sqrt%2813%29  and  x%5B2%5D = 5+-+sqrt%2813%29.