Thích không lập phương thì không lập phương. T dễ tính lắm
\(A=\sqrt[3]{5+2\sqrt{13}}+\sqrt[3]{5-2\sqrt{13}}\)
\(=\dfrac{1}{2}.\left(\sqrt[3]{40+16\sqrt{13}}+\sqrt[3]{40-16\sqrt{13}}\right)\)
\(=\dfrac{1}{2}.\left(\sqrt[3]{1+3\sqrt{13}+39+13\sqrt{13}}+\sqrt[3]{1-3\sqrt{13}+39-16\sqrt{13}}\right)\)
\(=\dfrac{1}{2}.\left(\sqrt[3]{\left(1+\sqrt{13}\right)^3}+\sqrt[3]{\left(1-\sqrt{13}\right)^3}\right)\)
\(=\dfrac{1}{2}.\left(1+\sqrt{13}+1-\sqrt{13}\right)=\dfrac{2}{2}=1\)
\(A=\sqrt[3]{5+2\sqrt{13}}+\sqrt[3]{5-2\sqrt{13}}\)
\(\sqrt[3]{5+2\sqrt{13}}=a\)
\(\sqrt[3]{5-2\sqrt{13}}=b\)
\(a^3+b^3=5+2\sqrt{13}+5-2\sqrt{13}=10\)
\(ab=\sqrt[3]{\left(5+2\sqrt{13}\right)\left(5-2\sqrt{13}\right)}=\sqrt[3]{25-52}=\sqrt[3]{-27}=-3\)
\(A^3=\left(a+b\right)^3=a^3+b^3+3ab\left(a+b\right)\)
\(A^3=10-9A\)
\(A^3+9a-10=0\)
\(\left(A-1\right)\left(A^2+A+10\right)=0\)
\(A^2+A+10>0\) mọi A
\(A-1=0\Rightarrow A=1\) là nghiệm duy nhất
KL: A = 1
