Monday, August 19, 2013

if alpha (α) and beta (β) are the roots of 2x^2-7x+10=0, find: (α) + (β) (α) x (β) (α)^2 + (β)^2 (α)^2 x (β) + (β)^2 x (α)

We'll find the roots of the quadratic using the
formula:


`x_1 = (-b+sqrt(b^2 -
4ac))/(2a)`


`x_2 = (-b - sqrt(b^2 -
4ac))/(2a)`


a,b,c are the coefficients of the
quadratic:


a = 2 ; b = -7 and c =
10


`x_1 = (7 + sqrt(49 -
80))/(4)`


We notice that the result of the difference under
the radical sign is negative, therefore the equation has complex
roots.


`x_1 = (7+isqrt31)/4` and `x_2 = (7 -
isqrt31)/4`


We'll put `x1 = alpha` and `x_2 =
beta`


`alpha + beta = (7 + isqrt31 + 7 -
isqrt31)/4`


We'll eliminate imaginary
parts:


`alpha + beta =
14/4`


`alpha + beta =
7/2`


We'll calculate the product of roots, that is a
special product which returns a difference of two
squares:


`alpha*beta = (49 -
31i^2)/16`


But `i^2 = -1` => `alpha*beta =
(49+31)/16` => `alpha*beta = 5`


We'll calculate
`alpha^2 + beta^2` :


`alpha^2 + beta^2 = (alpha + beta)^2 -
2alpha*beta`


`alpha^2 + beta^2 = 49/4 - 10` =>
`alpha^2 + beta^2 = 9/4`


We'll calculate `alpha^2*beta +
beta^2*alpha = alpha*beta(alpha + beta)`


`alpha^2*beta +
beta^2*alpha = 5*7/2 = 35/2`


Therefore, the
requested results are: `alpha + beta = 7/2 ; alpha*beta = 5 ; alpha^2 + beta^2 = 9/4 and
alpha^2*beta + beta^2*alpha = 35/2` .

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