If equation ax^2 + bx + c = 0 has roots Alpha and Beta, find values of these expressions in terms of a, b, and c. (1) alpha^4 + beta^4 (2) alpha^2/beta + beta^2/alpha (3) alpha^3/beta + beta^3/alpha (4) 1/(a*alpha + b) + 1/(a*beta + b)

Given that equation ax^2 + bx + c = 0 has roots α and β

=> α + β = -b/a
and αβ = c/a

Keeping these in mind, we'll now solve the problems.

1) α^4 + β^4
=> (α²)² + (β²)²
=> (α² + β²)² - 2α²β²        by using a² + b² = (a + b)² - 2ab
=> ((α + β)²  - 2αβ)² - 2(αβ)²        using the same identity again

We will now substitute the values of α + β and αβ we found in the beginning of the solution

=> ((-b/a)² - 2c/a)² - 2(c/a)²
=> (b²/a² - 2c/a)² - 2c²/a²
=> ((b² - 2ac)/a²)² - 2c²/a²
=> (b² - 2ac)²/a - 2c²/a²
=> ((b² - 2ac)² - 2a²c²) / a
=> (b⁴ - 4ab²c + 4a²c² - 2a²c²) / a
=> (b⁴ - 4ab²c + 2a²c²) / a

2) α^2/β + β^2/α
=> ³ + β³)/αβ
=> (α + β)(α² + β² - αβ) /αβ

Again, using a² + b² = (a + b)² - 2ab

=> (α + β)((α + β)² - 3αβ) / αβ
=> (-b/a)((-b/a)² - 3(c/a)) / (c/a)
=> (-b/a)(b²/a² - 3c/a) / (c/a)
=> (-b/c)((b² - 3ac)/ a²)
=> (-b³ + 3abc) / a²c 

3) α^3/β + β^3/α
=> + β)/αβ

We've already calculated for + β) in the first question. We will use the result here.

=> ((b⁴ - 4ab²c + 2a²c²) / a⁴) / αβ
=> ((b⁴ - 4ab²c + 2a²c²) / a⁴) / (c/a)
=> (b⁴ - 4ab²c + 2a²c²) / a³c

4) 1/(aα + b) + 1/(aβ + b)

We already know that 
α + β = -b/a
=> a(α + β) = -b
=> -a(α + β) = b

Substituting this value of b in the problem,

=> 1/(aα + b) + 1/(aβ + b)
=> 1/(aα - a(α + β)) + 1/(aβ - a(α + β))
=> 1/(-aβ) + 1/(-aα)
=> -1/a (1/β + 1/α)
=> -1/a (α + β)/αβ
=> -1/a (-b/a)(c/a)
=> b/ac    


8 comments:

  1. α+β=5 and α²+β²=19 show that αβ=3

    ReplyDelete
    Replies
    1. Use alha plus bheta whole square formula

      Delete
    2. α+β=5
      => (α+β)^2 = 25
      => α^2 + β^2 + 2αβ = 25
      => 19 + 2αβ = 25
      => 2αβ = 25 - 19
      => 2αβ = 6
      => αβ = 3

      Delete
  2. Same qquestio for beta/(a alpa+b) (alpha/a beta+b)

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