If a quadratic equation is given in standard form, we can find the sum and product of the roots using coefficient of x2, x and constant term.
Let us consider the standard form of a quadratic equation,
ax2 + bx + c = 0
(Here a, b and c are real and rational numbers)
Let α and β be the two zeros of the above quadratic equation.
Then the formula to get sum and product of the roots of a quadratic equation are,
Sum of roots (a + ᵦ) :
= -b/a
(or)
= -coefficient of x/coefficient of x2
Product of the roots (aᵦ) :
= c/a
(or)
= constant term/coefficient of x2
Find the sum and product of the roots for each of the following quadratic equations.
Question 1 :
x2 + 3x - 28 = 0
Solution :
Comparing x2 + 3x - 28 = 0 and ax2 + bx + c = 0, we get
a = 1, b = 3 and c = -28
Sum of the roots = -b/a
= -3/1
= -3
Product of the roots = c/a
= -28/1
= -28
Question 2 :
x2 + 3x = 0
Solution :
Comparing x2 + 3x = 0 and ax2 + bx + c = 0, we get
a = 1, b = 3 and c = 0
Sum of the roots = -b/a
= -3/1
= -3
Product of the roots (aᵦ) = c/a
= 0/1
= 0
Question 3 :
3 + (1/a) = 10/a2
Solution :
10/a2 - (1/a) - 3 = 0
(10 - a - 3a2)/a2 = 0
-3a2 - a + 10 = 0
In the above quadratic equation,
coefficient of squared term a = -3,
coefficient of a term = -1
constant term = 10
Sum of the roots = -b/a
= -(-1)/(-3)
= -1/3
Product of the roots = c/a
= 10/(-3)
= -10/3
Question 4 :
3y2 - y - 4 = 0
Solution :
Comparing 3y2 - y - 4 = 0 and ax2 + bx + c = 0,
a = 3, b = -1 and c = -4
Sum of the roots = -b/a
= -(-1)/3
= 1/3
Product of the roots = c/a
= -4/3
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