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In mathematics, the method of clearing denominators, also called clearing fractions, is a technique for simplifying an equation equating two expressions that each are a sum of rational expressions – which includes simple fractions.
The method of equating coefficients is often used when dealing with complex numbers. For example, to divide the complex number a + bi by the complex number c + di , we postulate that the ratio equals the complex number e+fi , and we wish to find the values of the parameters e and f for which this is true.
are solved using cross-multiplication, since the missing b term is implicitly equal to 1: =. Any equation containing fractions or rational expressions can be simplified by multiplying both sides by the least common denominator. This step is called clearing fractions.
Theorem — The number of strictly positive roots (counting multiplicity) of is equal to the number of sign changes in the coefficients of , minus a nonnegative even number. If b 0 > 0 {\displaystyle b_{0}>0} , then we can divide the polynomial by x b 0 {\displaystyle x^{b_{0}}} , which would not change its number of strictly positive roots.
The factor x 2 − 4x + 8 is irreducible over the reals, as its discriminant (−4) 2 − 4×8 = −16 is negative. Thus the partial fraction decomposition over the reals has the shape Thus the partial fraction decomposition over the reals has the shape
D 1 is x + 1; set it equal to zero. This gives the residue for A when x = −1. Next, substitute this value of x into the fractional expression, but without D 1. Put this value down as the value of A. Proceed similarly for B and C. D 2 is x + 2; For the residue B use x = −2. D 3 is x + 3; For residue C use x = −3.
Denoting the two roots by r 1 and r 2 we distinguish three cases. If the discriminant is zero the fraction converges to the single root of multiplicity two. If the discriminant is not zero, and |r 1 | ≠ |r 2 |, the continued fraction converges to the root of maximum modulus (i.e., to the root with the greater absolute value).
This allows us to require that the denominator of the objective function (+) equals 1. (To understand the transformation, it is instructive to consider the simpler special case with α = β = 0 {\displaystyle \alpha =\beta =0} .)