Given that x=2 is a root of p(x)=x^4-3x^3-18x^2+90x-100, find the other roots (real and nonreal).
By synthetic division, we get that p(x)=(x−2)(x3−x2−20x+50). By Rational Root Theorem, the possible roots of x3−x2−20x+50 are ±1,±2,±5,±10,±25, and ±50. Testing values, we get that x=−5 works, so by synthetic division, we get that p(x)=(x−2)(x+5)(x2−6x+10).
By the quadratic formula, the roots of x2−6x+10 are 6±√(−6)2−4⋅1⋅102⋅1=6±2i2=3±i. Thus, the roots of p(x) are −5,2,3+i,and 3−i.