First, we factor the denominator:
x3−4x=x(x2−4)=x(x−2)(x+2). Thus, we can express the rational function as:
x(x−2)(x+2)2x2−18x+12=xA+x−2B+x+2C. Multiplying both sides by x(x−2)(x+2) gives: 2x2−18x+12=A(x−2)(x+2)+Bx(x+2)+Cx(x−2). 2x2−18x+12=A(x2−4)+B(x2+2x)+C(x2−2x). 2x2−18x+12=Ax2−4A+Bx2+2Bx+Cx2−2Cx. Grouping like terms, we get:
2x2−18x+12=(A+B+C)x2+(2B−2C)x−4A. By comparing coefficients, we obtain the following system of equations:
A+B+C=2 2B−2C=−18 From the third equation, we get A=−3. Substituting A=−3 into the first equation, we get −3+B+C=2, so B+C=5. Dividing the second equation by 2, we get B−C=−9. Adding the equations B+C=5 and B−C=−9 gives 2B=−4, so B=−2. Since B+C=5, we have −2+C=5, so C=7. Therefore, A=−3, B=−2, and C=7. Substituting these values back into the partial fraction decomposition, we have:
x(x−2)(x+2)2x2−18x+12=x−3+x−2−2+x+27. So, the partial fraction decomposition is −x3−x−22+x+27.