The possible rational roots are of the form pqwhere p is a factor of the constant term, in this case -4and q is a factor of the highest order term, i.e. 2so possible roots arefactors of -4 are±1, ±2, ±4factors of 2 are ±1, ±2so our possible roots arex=±12, ±1, ±2, ±4
we plug these into f(x) to determine which are actually rootsand see that x=−2, −12, 2are the only actual roots, and as f(x) is degree 3 that is all 3 of themso f(x)=c(x+2)(x+12)(x−2), for some c∈Qc is the coefficient of the x3 term sof(x)=2(x+2)(x+12)(x−2)
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