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How can you create a quadratic equation that cannot be solved by factoring but can be...

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zuzuman | Student, Undergraduate | eNoter

Posted April 18, 2011 at 6:55 PM via web

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How can you create a quadratic equation that cannot be solved by factoring but can be solved using the quadratic formula?


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giorgiana1976 | College Teacher | Valedictorian

Posted April 18, 2011 at 7:19 PM (Answer #1)

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We'll use the irrational roots of an equation that always come in pairs. We also use the fact that a polynomial could be written as a product of linear factors, such as:

P(x) = a(x – x1)(x – x2) (in this case we've written a quadratic, which has 2 roots)

We'll put x1 = p + sqrt m => the other root is the conjugate of x1 => x2 = p – sqrt m.

P(x) = a(x – p - sqrt m)(x – p + sqrt m)

We'll remove the brackets:

P(x) = ax^2 – ax*(p - sqrtm) – ax(p+sqrtm) – (p^2 - m)
P(x) = ax^2 – 2p*a*x - (p^2 - m)

The quadratic equation requested is  ax^2 – 2p*a*x - (p^2 - m) = 0, whose roots are irrational.

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted April 18, 2011 at 8:57 PM (Answer #2)

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A quadratic equation can also have complex roots. In that case too it would not be possible to arrive at the solution by factoring; instead we would have to use the quadratic formula.

Complex roots of a quadratic equation are always complex conjugates. If one root is a+ ib, the other is a - ib.

The quadratic equation would be:

(x - (a + ib))(x - (a - ib)) = 0

=> (x - a - ib)(x - a + ib) = 0

=> (x - a)^2 - (ib)^2 = 0

=> x^2 + a^2 - 2ax - i^2b^2 = 0

=> x^2 + a^2 - 2ax + b^2 = 0

=> x^2 - 2ax + a^2 + b^2 = 0

The equation we get is of the form x^2 - 2ax + a^2 + b^2 = 0

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