Let a and b be the roots of x2−4x+5=0. Compute a3+a4b2+a2b4+b3.
Solution — click to reveal
by Vieta's formulas, a+b=4 and ab=5. Then a3+b3=(a+b)(a2−ab+b2)=(a+b)(a2+2ab+b2−3ab)=(a+b)((a+b)2−3ab)=4⋅(42−3⋅5)=4,and a4b2+a2b4=a2b2(a2+b2)=(ab)2((a+b)2−2ab)=52(42−2⋅5)=150,so a3+a4b2+a2b4+b3=154.