Mathematics

Mathematics

Problem

If α\alpha and β\beta are the roots of the quadratic equation 4x2+2x−1=04x^2 + 2x - 1 = 0 then the value of ∑r=1∞(αr+βr)\sum_{r=1}^{\infty} (\alpha^r + \beta^r) is : (a) 2 (b) 3 (c) 6 (d) 0
Intuitive explanation

The key is recognizing that αr+βr\alpha^r + \beta^r follows a recurrence relation you can build from Vieta's formulas. Since α\alpha and β\beta satisfy the original equation, they obey a linear recurrence—use this to express each power sum in terms of previous ones. Then sum the infinite series by treating it as a geometric-like pattern that telescopes or converges to a closed form.

Intuitive explanation

The key is recognizing that αr+βr\alpha^r + \beta^r follows a recurrence relation you can build from Vieta's formulas. Since α\alpha and β\beta satisfy the original equation, they obey a linear recurrence—use this to express each power sum in terms of previous ones. Then sum the infinite series by treating it as a geometric-like pattern that telescopes or converges to a closed form.

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