Question

[My Nguyễn]

Số nghiệm của phương trình \(\left(2\sqrt[3]{x}+3\right)\left(2\sqrt[3]{x}+5\right)=21\)là?

CHỈ CHO MÌNH CÁCH LÀM VỚI

Answer 1

[Vũ Quang Vinh]

Đặt \(2\sqrt[3]{x}+3=a\). Khi đó biểu thức trên trở thành: \(a\left(a+2\right)=21\)
Mà \(\hept{\begin{cases}\left(a+2\right)-a=2\\\left(a+2\right)+a=k\end{cases}\Rightarrow\hept{\begin{cases}a+2=\frac{k+2}{2}\\a=\frac{k-2}{2}\end{cases}}}\) ( với k là hằng số )
\(\Rightarrow a\left(a+2\right)=\frac{k-2}{2}\cdot\frac{k+2}{2}\)
\(\Rightarrow\frac{\left(k-2\right)\left(k+2\right)}{4}=21\)
\(\Rightarrow k^2-4=84\)
\(\Rightarrow k^2=88\)
\(\Rightarrow\hept{\begin{cases}k=\sqrt{88}=2\sqrt{22}\\k=-\sqrt{88}=-2\sqrt{22}\end{cases}}\)
TH1: Nếu k > 0 thì
\(\Rightarrow a=\frac{2\sqrt{22}-2}{2}=\frac{2\left(\sqrt{22}-1\right)}{2}=\sqrt{22}-1\)
Thế lại vào ta có:
\(2\sqrt[3]{x}+3=\sqrt{22}-1\)
\(\Rightarrow2\sqrt[3]{x}=\sqrt{22}-4\)
\(\Rightarrow\sqrt[3]{x}=\sqrt{\frac{11}{2}}-2\)
\(\Rightarrow x=\left(\sqrt{\frac{11}{2}}-2\right)^3\)
\(\Rightarrow x=\left(\sqrt{\frac{11}{2}}\right)^3-3\cdot\left(\sqrt{\frac{11}{2}}\right)^2\cdot2+3\cdot\sqrt{\frac{11}{2}}\cdot2^2-2^3\)
\(\Rightarrow x=\sqrt{\left(\frac{11}{2}\right)^2\cdot\frac{11}{2}}-3\cdot\frac{11}{2}\cdot2+3\cdot\sqrt{\frac{11}{2}}\cdot4-8\)
\(\Rightarrow x=\frac{11}{2}\sqrt{\frac{11}{2}}-33+12\sqrt{\frac{11}{2}}-8\)
\(\Rightarrow x=\left(\frac{11}{2}\sqrt{\frac{11}{2}}+12\sqrt{\frac{11}{2}}\right)-\left(33+8\right)\)
\(\Rightarrow x=\frac{35}{2}\sqrt{\frac{11}{2}}-41\)

Answer 2

[Vũ Quang Vinh]

TH2: Nếu k < 0 thì:
\(\Rightarrow a=\frac{-2\sqrt{22}-2}{2}=\frac{-2\left(\sqrt{22}+1\right)}{2}=-\left(\sqrt{22}+1\right)\)
Thế lại vào ta có:
\(2\sqrt[3]{x}+3=-\left(\sqrt{22}+1\right)\)
\(\Rightarrow2\sqrt[3]{x}=-\left(\sqrt{22}+4\right)\)
\(\Rightarrow\sqrt[3]{x}=-\left(\sqrt{\frac{11}{2}}+2\right)\)
\(\Rightarrow x=-\left(\sqrt{\frac{11}{2}}+2\right)^3\)
\(\Rightarrow x=-\left[\left(\sqrt{\frac{11}{2}}\right)^3+3\cdot\left(\sqrt{\frac{11}{2}}\right)^2\cdot2+3\cdot\sqrt{\frac{11}{2}}\cdot2^2+2^3\right]\)
\(\Rightarrow x=-\left[\sqrt{\left(\frac{11}{2}\right)^2\cdot\frac{11}{2}}+3\cdot\frac{11}{2}\cdot2+3\cdot\sqrt{\frac{11}{2}}\cdot4+8\right]\)
\(\Rightarrow x=-\left[\left(\frac{11}{2}\sqrt{\frac{11}{2}}+12\sqrt{\frac{11}{2}}\right)+\left(33+8\right)\right]\)
\(\Rightarrow x=-\left[\frac{35}{2}\sqrt{\frac{11}{2}}+41\right]\)
\(\Rightarrow x=-\frac{35}{2}\sqrt{\frac{11}{2}}-41\)