Saturday, October 9, 2010

Use partial fractions(3x-2)/(x-3)(x+1)

We'll decompose the given rational function in the
elementary quotients:


(3x-2)/(x-3)(x+1) = A/(x-3) +
B/(x+1)


We notice that the numerator of the original ratio
is a linear function and the denominator is a quadratic
function.


The irreducible ratios from the right side have
as numerators constant functions and as denominators, linear
functions.


We'll calculate LCD of the 2 ratios from the
right side.


The LCD is the same with the denominator from
the left side.


LCD =
(x-3)(x+1)


The expression will
become:


(3x-2) = A(x+1) +
B(x-3)


We'll remove the
brackets:


3x - 2 = Ax + A + Bx -
3B


We'll combine like terms form the right
side:


3x - 2 = x(A+B) +
(A-3B)


If the expressions from both sides are equal, then
the correspondent coefficients are equals:


3 =
A+B


-2 = A - 3B


We'll use the
symmetric property:


A+B = 3
(1)


A - 3B = -2 (2)


We'll
multiply (1) by 3:


3A+3B = 9
(3)


We'll add (3) to
(2):


3A+3B+A - 3B = 9-2


We'll
eliminate like terms:


4A =
7


We'll divide by 4:


A =
7/4


We'll substitute A in
(1):


A+B = 3


7/4 + B =
3


B = 3 - 7/4


B =
(12-7)/4


B =
5/4


(3x-2)/(x-3)(x+1) = 74/(x-3) +
5/4(x+1)

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