Without using the root formula, find the roots of the quadratic equation x ^ 2-5x + 4 = 0, x ^ 2-6x-16 = 0.
1) Equation x ^ 2 – 5x + 4 = 0:
Let’s sum up all the coefficients of the equation:
1 + (-5) + 4 = 0.
A result of 0 means that one of the roots is 1.
The product of two roots of the equation is equal to a constant:
x1 * x2 = s.
Let’s find the second root from this formula:
1 * x2 = 4;
x2 = 4.
2) Equation x ^ 2 – 6x – 16 = 0.
Let’s sum up all the coefficients of the equation:
1 + (-6) + (-16) = -21.
The sum of two roots is equal to the coefficient b with the opposite sign:
x1 + x2 = -b.
We now have two formulas for the roots of the equation, which form a system of equations. Let’s find both roots from it:
x1 + x2 = – (-6);
x1 * x2 = -16.
You can solve the system by substituting x1 through x2, or you can pick up the roots in your head.
After selection, we get:
x1 = -2;
x2 = 8.
Answer: 1) x1 = 1; x2 = 4.2) x1 = -2; x2 = 8.