7-3-33 partial fraction linear to a power and quadratic
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For this problem we need to have a / X + b / X ^2.
Because X ^2 is a linear term, we need every power up until
whatever the power that the linear term is 2.
But then we also are going to have acx plus d / X ^2 + 4.
The reason for this is the X ^2 + 4 is factored as much as it
can be and it's quadratic.
So above the quadratic has got to be a linear.
So now when we multiply through by the common denominator, we're
going to get X + 7 equaling axx squared plus 4 + b * X ^2 + 4
plus X ^2 times the CX plus D So if I have X + 7, that's going to
equal AX cubed plus 4AX plus BX squared plus 4B plus CX cubed
plus DX squared.
So the cube terms AX cubed and CX cubed have got to equal the
cubed terms on the other side.
So we know that 0 = a + C The X squared terms on the one side
have to equal the X ^2 terms on the other.
So zero is going to equal b + d The X term on one side has got
to equal the X term on the other.
So the coefficient of one is going to equal 4A and then the
constant has to equal the constant.
Now a way to check it is to count how many terms you have
123456 and just make sure we had six here so that we didn't lose
any.
Well, we can see that if one equal 4A, a is 1/4 and if A is
1/4 and we know that A + C = 0, then C has got to be the
opposite of A or negative 1/4.
We can see B is going to be 7 fourths and if B + D has to
equal 0 then D's got to be the opposite or -7 fourths.
So we're going to have one over 4X plus seven over four, X ^2 +
-1 fourth, X -7 fourths all over X ^2 + 4.
Now we can make that into a proper fraction by multiplying
the top and the bottom each by 4.
So we'd get one over four X + 7 / 4 X squared plus negative X -,
7 / 4 times the quantity X ^2 + 4.
So that looks like that's going to be answer D1 over four X + 7
/ 4 X squared plus negative X - 7 / 4 * X ^2 + 4.