Giải thích các bước giải:
Câu 1:
1.Ta có $A=\sqrt{20}-\sqrt{45}+3\sqrt{18}+\sqrt{72}$
$\to A=2\sqrt{5}-3\sqrt{5}+9\sqrt{2}+6\sqrt{2}$
$\to A=15\sqrt{2}-\sqrt{5}$
2.Ta có :
$P=\dfrac{\sqrt{x}+1+x+\sqrt{x}}{\sqrt{x}+1}.\dfrac{\sqrt{x}-1-x+\sqrt{x}}{\sqrt{x}-1}$
$\to P=\dfrac{x+2\sqrt{x}+1}{\sqrt{x}+1}.\dfrac{-(x-2\sqrt{x}+1)}{\sqrt{x}-1}$
$\to P=\dfrac{(\sqrt{x}+1)^2}{\sqrt{x}+1}.\dfrac{-(\sqrt{x}-1)^2}{\sqrt{x}-1}$
$\to P=-(\sqrt{x}+1)(\sqrt{x}-1)=-x+1$
Câu 2:
a.$\sqrt{2x}=8\to 2x=64\to x=32$
b.$\sqrt{x-1}+\sqrt{4x-4}+\sqrt{9x-9}-\dfrac{1}{2}\sqrt{16x-16}=2$
$\to\sqrt{x-1}+2\sqrt{x-1}+3\sqrt{x-1}-2\sqrt{x-1}=2$
$\to 4\sqrt{x-1}=2$
$\to\sqrt{x-1}=\dfrac 12$
$\to x=\dfrac 54$
