The positive difference between the two roots of the quadratic equation $3x^2 - 7x - 8 = 0$ can be written as $\frac{\sqrt{m}}{n}$, where $n$ is an integer and $m$ is an integer not divisible by the square of any prime number. Find $m + n$.

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DeanR

3x² - 7x - 8 = 0

We're asked about square roots so we won't try to factor; we'll go right for the quadratic formula,

x = ( 7 ± √(7² - 4(3)(-8))    )/(2(3)) = (7 ± √(49+96))/6 = 7/6 ± √145/6

145 = 5×29, so no square factors.  The positive difference is

d =  (7/6 + √145/6) - (7/6 - √145/6) = 2√145/6 = √145/3

so m=145, n=3 for a sum of

Answer: 148

3x² - 7x - 8 = 0

We're asked about square roots so we won't try to factor; we'll go right for the quadratic formula,

x = ( 7 ± √(7² - 4(3)(-8))    )/(2(3)) = (7 ± √(49+96))/6 = 7/6 ± √145/6

145 = 5×29, so no square factors.  The positive difference is

d =  (7/6 + √145/6) - (7/6 - √145/6) = 2√145/6 = √145/3

so m=145, n=3 for a sum of

Answer: 148

What is a Quadratic Equation?

A quadratic equation is an algebraic equation of the second degree in x. The quadratic equation in its standard form is ax2 + bx + c = 0, where a and b are the coefficients, x is the variable, and c is the constant term.

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