Squares And Quadratics

Algebra Level 2

x 2 + p x + q = 0 \large x^2 + px + q = 0

Let p p and q q be integers such that the equation above has 2 distinct integer roots. Which of the following must be a perfect square ?

p 2 2 q p^2-2q p 2 3 q p^2-3q p 2 4 q p^2-4q p 2 5 q p^2-5q

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4 solutions

Since the roots ae integers , so discriminant = p 2 4 q p^2-4q is a perfect square.

For mathematical experiment for x 2 + p x + q = 0 x^2+px+q=0 . By using the completing the square we can determine the perfect square. So the perfect square for x 2 + p x + q = 0 x^2+px+q=0 is p 2 4 q p^2-4q that is a discriminant.

ADIOS!!! \LARGE \text{ADIOS!!!}

For the quadratic equation a x 2 + b x + c = 0 ax^2 + bx + c = 0 to contain integer roots, its determinant b 2 4 a c \sqrt{b^2 - 4ac} must be a perfect square. In the quadratic equation above where p and q are integers, we can substitute its values for a, b, and c into the discriminant.

For x 2 + p x + q x^2 + px + q , the discriminant can be expressed as: p 2 4 q \sqrt{p^2 - 4q} . Thus, p 2 4 q p^2 - 4q must be a perfect square.

Sahil Nare
Apr 1, 2016

-p = a + b, p^2 = a^2 + 2ab + b^2, q = ab, 4q = 4ab, p^2 - 4q = a^2 + 2ab + b^2 - 4ab, p^2 - 4q = a^2 - 2ab + b^2

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