Maths-
General
Easy

Question

Find the extraneous solution of square root of 8 x plus 9 end root equals x
 

hintHint:

Rearrange the equation and then solve for x.

The correct answer is: ⇒ x = -1


    Complete step by step solution:
    Here we have the equation square root of 8 x plus 9 end root equals x
    Now, on squaring both the sides, we get left parenthesis square root of 8 x plus 9 end root right parenthesis squared equals x squared

    not stretchy rightwards double arrow 8 x plus 9 equals x squared
    On rearranging, we have x squared minus 8 x minus 9 equals 0
    Now, this is a quadratic equation with a = 1, b = -8, c = -9
    Roots can be found with, x equals fraction numerator negative b plus-or-minus square root of b squared minus 4 a c end root over denominator 2 a end fraction

    x equals fraction numerator 8 plus-or-minus square root of 64 minus 4 cross times 1 cross times negative 9 end root over denominator 2 cross times 1 end fraction
    On solving, we get x = 9, -1
    Here, on substituting x = 9 or -1 in given equation we get

    square root of 8 cross times 9 plus 9 end root equals 9
    not stretchy rightwards double arrow square root of 72 plus 9 end root equals square root of 81 equals 9
    Hence x = 9 works.

    Now, equals square root of 8 cross times negative 1 plus 9 end root equals negative 1
    not stretchy rightwards double arrow square root of negative 8 plus 9 end root equals square root of 1 not equal to negative 1
    Hence x = -1 is an extraneous solution.

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