5.4-bo'lim : ko'phadlarning nollarini topish
1. Quyidagi ko‘phadning barcha nollarini toping.
\[f\left( x \right) = 2{x^3} - 13{x^2} + 3x + 18\]Yechim
Bizga 18 va 2 ning barcha omillari kerak bo'ladi.
\[\begin{align*}18 & : \pm 1, \pm 2, \pm 3, \pm 6, \pm 9, \pm 18\\ 2 & : \pm 1,\,\, \pm 2\end{align*}\]2-qadam
Bu ko'phad uchun barcha mumkin bo'lgan ratsional nollarning ro'yxati.
\[\begin{align*}\frac{{ \pm 1}}{{ \pm 1}} & = \pm 1 & \hspace{0.25in}\frac{{ \pm 2}}{{ \pm 1}} & = \pm 2 & \hspace{0.25in}\frac{{ \pm 3}}{{ \pm 1}} & = \pm 3 & \hspace{0.25in}\frac{{ \pm 6}}{{ \pm 1}} & = \pm 6 & \hspace{0.25in}\frac{{ \pm 9}}{{ \pm 1}} & = \pm 9 & \hspace{0.25in}\frac{{ \pm 18}}{{ \pm 1}} & = \pm 18\\ & \\ \frac{{ \pm 1}}{{ \pm 2}} & = \frac{{ \pm 1}}{{ \pm 2}} & \hspace{0.25in}\frac{{ \pm 2}}{{ \pm 2}} & = \pm 1 & \hspace{0.25in}\frac{{ \pm 3}}{{ \pm 2}} & = \frac{{ \pm 3}}{{ \pm 2}} & \hspace{0.25in}\frac{{ \pm 6}}{{ \pm 2}} & = \pm 3 & \hspace{0.25in}\frac{{ \pm 9}}{{ \pm 2}} & = \frac{{ \pm 9}}{{ \pm 2}} & \hspace{0.25in}\frac{{ \pm 18}}{{ \pm 2}} & = \pm 9\end{align*}\]Demak, ko'phad uchun jami 18 ta nol mavjud.
3-qadam
Endi biz sintetik bo'linish ishini boshlashimiz kerak. Biz birinchi navbatda "kichik" butun sonlardan boshlaymiz.
\[\,\,\begin{array}{r|rrrl} {} & 2 & -13 & 3 & 18 \\ \hline -1 & 2 & -15 & 18 & 0=f\left( -1 \right)=0!! \\ \end{array}\]Endi biz \(x = - 1\) nol ekanligini bilamiz va ko'phadni quyidagicha yozishimiz mumkin:
\[f\left( x \right) = 2{x^3} - 13{x^2} + 3x + 18 = \left( {x + 1} \right)\left( {2{x^2} - 15x + 18} \right)\]4-qadam
Biz bu jarayonni davom ettirishimiz mumkin, ammo bizda ikkinchi omil uchun kvadratik bor va biz buni shunchaki faktorlashimiz mumkin, shuning uchun ko'phadning to'liq faktorlangan shakli,
\[f\left( x \right) = 2{x^3} - 13{x^2} + 3x + 18 = \left( {x + 1} \right)\left( {2x - 3} \right)\left( {x - 6} \right)\]5-qadam
To'liq faktorlangan shakldan biz asl ko'phad uchun quyidagi nollar to'plamini olamiz.
\[\require{bbox} \bbox[2pt,border:1px solid black]{{x = - 1\hspace{0.25in}\hspace{0.25in}\hspace{0.25in}x = \frac{3}{2}\hspace{0.25in}\hspace{0.25in}\hspace{0.25in}x = 6}}\]