If α and β are the Zeros of the Quadratic Polynomial f(x) = ax2 + bx + c, then Evaluate
By BYJU'S Exam Prep
Updated on: October 17th, 2023
If α and β are the zeroes of the quadratic polynomial f(x) = ax2 + bx + c, then evaluate:
(i) α − β
(ii) 1/α − 1/β
(iii) 1/α + 1/β – 2αβ
(iv) α2β − αβ2
(v) α4 + β4
If α and β are the zeroes of the quadratic polynomial f(x) = ax2 + bx + c, then we know the following basic things which can be used to find the desired results:
- α + β = -b/a
- αβ = c/a
- Quadratic polynomial can be factored as f(x) = a(x – α)(x – β)
Table of content
Solution:
(i) α − β
To find α – β, we need to know the values of α and β, which are the zeroes of the quadratic polynomial. Let’s assume the quadratic polynomial is f(x) = ax2 + bx + c, and its zeroes are α and β.
Then we know that:
α + β = -b/a
We can rearrange this equation to get:
α = -b/a – β
Now, we substitute this expression for α into the equation f(α) = 0, which gives:
aα2 + bα + c = 0
Substituting for α, we get:
a(-b/a – β)2 + b(-b/a – β) + c = 0
Expanding and simplifying, we get:
aβ2 + (-2ab/a)β + (a/b)c – b/a = 0
Multiplying both sides by b/a, we get:
abβ2 – 2abβ + ac – b2 = 0
Dividing both sides by ab, we get:
β2 – (b/a)β + (c/a) = 0
This is a quadratic equation in β, with coefficients -b/a and c/a. So we can use the quadratic formula to solve for β:
β = [-(b/a) ± √((b/a)2 – 4(c/a))]/2
Once we have found the values of α and β, we can easily find α – β:
α – β = (-b/a – β) – β
α – β = -b/a – 2β
α – β = -b/a – 2[(-b/a) ± √((b/a)2 – 4(c/a))]/2
Simplifying, we get:
α – β = ±√(b2 – 4ac)/a
(ii) 1/α − 1/β
1/α – 1/β = (β – α)/(αβ)
Using the quadratic formula, we have:
α,β = [-b ± sqrt(b2 – 4ac)]/(2a)
So, substituting the expression for α and β:
1/α – 1/β = [(b + sqrt(b2 – 4ac)) – (b – sqrt(b2 – 4ac))]/[2a(sqrt(b2 – 4ac))]
Simplifying the numerator:
1/α – 1/β = 2sqrt(b2 – 4ac)/[2a(sqrt(b2 – 4ac))]
1/α – 1/β = ±sqrt(b2 – 4ac)/c
Therefore, the correct answer for (ii) in terms of a, b, and c is:
1/α – 1/β = ±sqrt(b2 – 4ac)/c
(iii) 1/α + 1/β – 2αβ
We can first find a common denominator for the two fractions by multiplying them by (αβ)/(αβ):
1/α + 1/β – 2αβ = (β + α) / (αβ) – 2αβ
= (β + α – 2αβ²) / (αβ)
using αβ = c/a and α + β = -b/a
1/α + 1/β – 2αβ = (-b/a) / (c/a) – 2c/a
On solving we will get,
= -(b/c + 2c/a)
Therefore, 1/α + 1/β – 2αβ = -b/c – 2c/a.
(iv) α2β − αβ2
We can rewrite the expression α2β − αβ2 as αβ(α − β)
On putting αβ = c/a and α − β = -b/a, we get
= c/a(− b/a)
= -cb/a2
(v) α4 + β4
Simplifying: α4 + β4 = (α2 + β2)2 – 2α2β2
We already know that:
α + β = -b/a
αβ = c/a
Using these values, we can find:
α2 + β2 = (α + β)2 – 2αβ
α2 + β2 = (b/a)2 – 2(c/a)
α2 + β2 = (b2 – 2ac)/a2
Substituting this back into the original identity, we get:
α4 + β4 = ((b2 – 2ac)/a2)2 – 2α2β2
α4 + β4 = (b4 – 4b2ac + 4a2c2)/a4 – 2(c/a)2
α4 + β4 = (b4 – 4b2ac + 4a2c2 – 2c2a2)/a4
Simplifying this expression, we get:
α4 + β4 = (b4 – 4b2ac + 4a2c2 – 2c2a2)/a4
α4 + β4 = (b4 – 4b2ac + 4c2(a2 – c))/a4
Therefore, the value of α4 + β4 in terms of a, b, and c is:
α4 + β4 = (b4 – 4b2ac + 4c2(a2 – c))/a4
or
α4 + β4 = (b2 – 2ac)2– 2a2c2))/a4
If α and β are the Zeros of the Quadratic Polynomial f(x) = ax2 + bx + c, then Evaluation Answers
(i) α − β = ±sqrt(b2 – 4ac)/a
(ii) 1/α − 1/β = ±sqrt(b2 – 4ac)/c
(iii) 1/α + 1/β – 2αβ = -b/c – 2c/a
(iv) α2β − αβ2 = -cb/a2
(v) α4 + β4 = (b2 – 2ac)2– 2a2c2))/a4
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