Saturday, August 26, 2017

`3/(x^3 + x - 2)` Write the partial fraction decomposition of the rational expression. Check your result algebraically.

You need to decompose the fraction into irreducible fractions, such that:


`(3)/(x^3+x-2) = (3)/(x^3+x-1-1) = 3/((x^3-1) + (x-1))`


`3/((x^3-1) + (x-1)) = 3/((x-1)(x^2+x+1) + (x-1))`


`3/((x-1)(x^2+ x + 2)) = A/(x-1) + (Bx+C)/(x^2+ x + 2)`


You need to bring the fractions to a common denominator:


`3 = Ax^2 + Ax + 2A + Bx^2 - Bx + Cx - C`


You need to group the terms having the same power of x:


`3 = x^2(A+B)...

You need to decompose the fraction into irreducible fractions, such that:


`(3)/(x^3+x-2) = (3)/(x^3+x-1-1) = 3/((x^3-1) + (x-1))`


`3/((x^3-1) + (x-1)) = 3/((x-1)(x^2+x+1) + (x-1))`


`3/((x-1)(x^2+ x + 2)) = A/(x-1) + (Bx+C)/(x^2+ x + 2)`


You need to bring the fractions to a common denominator:


`3 = Ax^2 + Ax + 2A + Bx^2 - Bx + Cx - C`


You need to group the terms having the same power of x:


`3 = x^2(A+B) + x(A - B + C) + 2A - C`


Comparing both sides yields:


`A+B =0 => A = -B`


A - B + C = 0 => 2A + C = 0


`2A - C = 3`


Adding the relations yields:


`4A =3 => A = 3/4 => B = -3/4 => C = -6/4`


Hence, the partial fraction decomposition is `3/((x-1)(x^2+ x + 2)) = 3/(4x-4) + (-3x-6)/(4x^2+ 4x + 8).`

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