If $\frac{x^{4}+24 x^{2}+28}{\left(x^{2}+1\right)^{3}}=\frac{A}{x^{2}+1}+\frac{B}{\left(x^{2}+1\right)^{2}}$ $+\frac{C}{\left(x^{2}+1\right)^{3}}$ $\Rightarrow x^{4}+24 x^{2}+28=A\left(x^{2}+1\right)^{2}+B\left(x^{2}+1\right)+C$ On comparing the coefficient of different terms $A=1;\, 2 A+B=24$ and $A+B+C=28$ $\Rightarrow A=1,\, B=22$, so $A+C=6$
The number of formulas used to decompose the given improper rational functions is given below. By using the given expressions, we can quickly write the integrand as a sum of proper rational functions.