Write the fraction to be decomposed
x3−3x−22x+3 We start from the given rational function.
Factorise the denominator
x3−3x−2=(x−2)(x+1)2 Factorising shows which linear factors appear.
Write the partial fraction form with unknown constants
(x−2)(x+1)22x+3=x+1A+(x+1)2B+x−2C Each distinct factor (and each power of a repeated factor) needs its own term.
Cover up \left(x + 1\right)^{2} and substitute x=-1
1=B(−3) ⇒ B=−31 Multiplying by the factor and substituting its root isolates one constant.
Cover up x - 2 and substitute x=2
7=C(9) ⇒ C=97 Multiplying by the factor and substituting its root isolates one constant.
Substitute x=0 to find A
x=0: A=−97 With the other constants known, any convenient value gives the last one.
Multiply through by the denominator to form an identity
2x+3=A(x2−x−2)+B(x−2)+C(x2+2x+1) Clearing the fractions gives an identity true for all x.
Expand the right-hand side
2x+3=Ax2−Ax−2A+Bx−2B+Cx2+2Cx+C Expanding lets us compare like terms.
Compare coefficients of like powers of x
A+C=0, −A+B+2C=2, −2A−2B+C=3 Matching coefficients confirms the values of the constants.
Express the assumed form over a common denominator
x+1A+(x+1)2B+x−2C=x3−3x−2A(x2−x−2)+B(x−2)+C(x2+2x+1) Recombining shows the numerators must match.
Verify by recombining the partial fractions
−9(x+1)7−3(x+1)21+9(x−2)7=(x−2)(x+1)22x+3 Adding the fractions back returns the original expression.
Check the result at a test value
x=0: −23=−23 Both sides agree, confirming the decomposition.
Note the degrees involved
degN=1, degD=3 The degree comparison determines whether division is needed.
Recall the cover-up method
Multiply by a factor, then substitute its root. The cover-up method is the quickest route to each constant.
Select the correct decomposition
(x−2)(x+1)22x+3=−9(x+1)7−3(x+1)21+9(x−2)7 This is the fully correct partial fraction decomposition.