Câu 20: \(\sqrt[3]{8x}+\sqrt[3]{27x}=10\)
=>\(2\cdot\sqrt[3]{x}+3\cdot\sqrt[3]{x}=10\)
=>\(5\cdot\sqrt[3]{x}=10\)
=>\(\sqrt[3]{x}=2\)
=>x=8
=>Chọn A
Câu 19: A
câu 18:
ĐKXĐ: x>=1
Đặt A=\(\sqrt{x+2\sqrt{x-1}}+\sqrt{x-2\sqrt{x-1}}\)
\(=\sqrt{x-1+2\cdot\sqrt{x-1}\cdot1+1}+\sqrt{x-1-2\cdot\sqrt{x-1}\cdot1+1}\)
\(=\sqrt{\left(\sqrt{x-1}+1\right)^2}+\sqrt{\left(\sqrt{x-1}-1\right)^2}=\left|\sqrt{x-1}+1\right|+\left|\sqrt{x-1}-1\right|\)
=>\(A\ge\sqrt{x-1}+1+1-\sqrt{x-1}=2\forall x\) thỏa mãn ĐKXĐ
=>Chọn A
Câu 17: A
Câu 16: C
Câu 15: B
Câu 14: \(\left(1+\sqrt3+\sqrt2\right)\left(1+\sqrt3-\sqrt2\right)\)
\(=\left(1+\sqrt3\right)^2-2\)
\(=4+2\sqrt3-2=2\sqrt3+2\)
=>Chọn B
