Quadratic Equations: Deriving New Equations from Modified Roots

Rarely Tested

Finding a quadratic equation:

  • If roots are reciprocals of roots of equation $$ax^2 + bx + c = 0$$, then equation is $$cx^2 + bx + a = 0$$

  • If roots are k more than roots of $$ax^2 + bx + c = 0$$ then equation is $$a(y-k)^2 + b(y-k) + c = 0$$

  • If roots are k times roots of $$ax^2 + bx + c = 0$$ then equation is $$a(y/k)^2 + b(y/k) + c = 0$$

  • If roots are negatives of roots of ax²+bx+c=0, new equation is ax²−bx+c=0

  • If p and q are roots of ax²+bx+c=0, and we want a new equation whose roots are p² and q². The equation will be $$a^2x^2−(b^2−2ac)x+c^2=0$$

Question 1

Amrit solves a quadratic equation but notes the constant term wrong and gets the roots as (9,4) while Nag notes the coefficient of x wrong and gets the roots as (6,2). What are the correct roots of the original equation?

Question 2

If 'a' and 'b' are the roots of the equations $$x^{2} - 9x + 3=0$$, find the quadratic equation whose roots are $$\frac{a}{b}$$ and $$\frac{b}{a}$$

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