Let a,b, and c be the roots of x3−7x2+5x+2=0. Find bc+1a+ac+1b+ab+1c.
Solution — click to reveal
By Vieta's formulas, a+b+c=7,ab+ac+bc=5, and abc=−2.
We can say bc+1a+ac+1b+ab+1c=abc+aa2+abc+bb2+abc+cc2.Since abc=−2, this becomes a−2a2+b−2b2+c−2c2.By Long Division, x−2x2=x+2+x−24, so a−2a2+b−2b2+c−2c2=a+2+a−24+b+2+b−24+c+2+c−24=a+b+c+6+4(a−21+b−21+c−21)=7+6+4⋅(a−2)(b−2)(c−2)(b−2)(c−2)+(a−2)(c−2)+(a−2)(b−2)=13+4⋅abc−2(ab+ac+bc)+4(a+b+c)−8(ab+ac+bc)−4(a+b+c)+12=13+4⋅−2−2⋅5+4⋅7−85−4⋅7+12=215.