Math 182 Miniproject 1 Partial Fractions.md
---
Math 182 Miniproject 1 Partial Fractions
===

**Overview:** In this project we explore more advanced partial fraction decomposition than we covered during class.

**Prerequisites:** Section 5.5 of _Active Calculus_ and a strong background in solving systems of linear equations.

For this miniproject we will need to know the general theory of partial fraction decompositions. We can rewrite a rational function $\frac{f(x)}{g(x)}$ by factoring $g(x)$ and looking at the powers of unique factors.

| Factor of $g(x)$ | Term in partial fraction |
| -------- | -------- |
| $ax+b$ | $\frac{A}{ax+b}$ |
| $(ax+b)^k$ | $\frac{A_1}{ax+b}+\frac{A_2}{(ax+b)^2}+\cdots+\frac{A_k}{(ax+b)^k}$ |
| $ax^2+bx+c$ | $\frac{Ax+B}{ax^2+bx+c}$ |
| $(ax^2+bx+c)^k$ | $\frac{A_1x+B_1}{ax^2+bx+c}+\frac{A_2x+B_2}{(ax^2+bx+c)^2}+\cdots+\frac{A_kx+B_k}{(ax^2+bx+c)^k}$ |

If the degree of $f(x)$ is greater than or equal to the degree of $g(x)$, then we have to do long division before finding the partial fraction decomposition.

__Example.__ The fraction $$\frac{4x^4+34x63+71x^2-32x-128}{x^2(x+4)^3}$$ has a partial fraction decomposition of the form $$
\frac{A}{x}+\frac{B}{x^2}+\frac{C}{x+4}+\frac{D}{(x+4)^2}+\frac{E}{(x+4)^3}.
$$

__Example.__ The fraction $$\frac{x^6+x^4+x^3-x^2-1}{x^3(x^2+1)^2}$$ has a partial fraction decomposition of the form $$
\frac{A}{x}+\frac{B}{x^2}+\frac{C}{x^3}+\frac{Dx+E}{x^2+1}+\frac{Fx+G}{(x^2+1)^2}.
$$
___

__Problem 1.__
Find the partial fraction decomposition of the function $$f(x)=\frac{4}{x^2(x^2+4)}$$.

$$
\frac{A}{x^2}+\frac{B}{x}+\frac{Cx+D}{x^2+4}$$

multiply by $x^2(x^2+4)$

$$4=A(x^2+4)+Bx(x^2+4)+(Cx+D)x^2$$

$x=0$ $$4=A\left(0^{2}+4\right)+B\left(0\right)\left(0^{2}+4\right)+\left(C\left(0\right)+D\right)\left(0^{2}\right) \rightarrow A=1$$

$x= 4i$ $$4=A\left(4i^{2}+4\right)+B\left(4i\right)\left(4i^{2}+4\right)+\left(C\left(4i\right)+D\right)\left(4i^{2}\right) \rightarrow D=-1$$

$x=2i,-2i$

$$4=A\left(2i^{2}+4\right)+B\left(2i\right)\left(2i^{2}+4\right)+\left(C\left(2i\right)+D\right)\left(2i^{2}\right) \rightarrow B, C=0$$
Thus, we get that $A=1,\ B=0,\ C=0,\ D=\left(-1\right)$

Therefore,
$$\frac{4}{x^2(x^2+4)}=\frac{A}{x^{2}}+\frac{B}{x}+\frac{\left(Cx+D\right)}{x^{2}+4}$$

$$\downarrow$$

$$\frac{4}{x^2(x^2+4)}=\frac{1}{x^{2}}+\frac{0}{x}+\frac{\left(0x+\left(-1\right)\right)}{x^{2}+4}$$

Answer:
$$\frac{4}{x^2(x^2+4)}=\frac{1}{x^{2}}+\frac{-1}{x^{2}+4}$$

___

__Problem 2.__
For the function $$g(x)=\frac{1}{(x+1)^4(x^2+1)}$$ write the form of the partial fraction decomposition. __Do not find the full partial fraction decomposition__.

$$
\frac{A}{(x+1)^4}+\frac{B}{(x+1)^3}+\frac{C}{(x+1)^2}+\frac{D}{(x+1)}+\frac{Ex+F}{(x^2+1)}
$$

___

__Problem 3.__
For the function $$h(x)=\frac{x^7}{(x^4-16)^2}$$ write the form of the partial fraction decomposition. __Do not find the full partial fraction decomposition__.

$$
\frac{Ax+B}{(x^2+4)^2}+\frac{Cx+D}{(x^2+4)}+\frac{E}{(x+2)^2}+\frac{F}{(x+2)}+\frac{G}{(x-2)^2}+\frac{H}{(x-2)}
$$

___

To submit this assignment click on the __Publish__ button. Then copy the url of the final document and submit it in Canvas.