Multiple choice

If the sum of the roots of the quadratic equation $\displaystyle ax^{2}+bx+c=0$ is equal to the sum of the squares of their reciprocals then $\displaystyle \frac{b^{2}}{ac}+\frac{bc}{a^{2}}$ is equal to

  1. $2$
  2. $-2$
  3. $1$
  4. $-1$
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A Correct answer
Explanation

Sum of roots = -b/a. Sum of squares of reciprocals = (1/r1)^2 + (1/r2)^2 = (r1^2 + r2^2) / (r1*r2)^2 = ((r1+r2)^2 - 2r1r2) / (r1r2)^2. Equating -b/a = (b^2/a^2 - 2c/a) / (c^2/a^2) leads to -b/a = (b^2 - 2ac) / c^2. Rearranging yields b^2/ac + bc/a^2 = 2.

AI explanation

Let the roots of the quadratic equation be $\alpha$ and $\beta$. Using Vieta's formulas, their sum is $\alpha + \beta = -b/a$, and their product is $\alpha\beta = c/a$. The given condition requires the sum of the roots to equal the sum of the squares of their reciprocals, so we substitute the values to get $-b/a = ((\alpha + \beta)^2 - 2\alpha\beta) / (\alpha\beta)^2$. Simplifying the right side yields $(b^2/a^2 - 2c/a) / (c^2/a^2)$, which reduces to $(b^2 - 2ac) / c^2$. Equating both sides gives $-bc^2 = a(b^2 - 2ac)$, which rearranges to $2a^2c = b^2a + bc^2$. Dividing the entire equation by $a^2c$ results in the value 2.