Đáp án:1)$\sqrt{7}+\sqrt{5}$
2a)1-x
b)$\sqrt{2}$
Giải thích các bước giải:
1)$\sqrt{112}-\sqrt{45}-\sqrt{63}+2\sqrt{20}$
=$\sqrt{16·7}-\sqrt{9·5}-\sqrt{9·7}+2\sqrt{4·5}$
=$4\sqrt{7}-3\sqrt{5}-3\sqrt{7}+4\sqrt{5}$
=$\sqrt{7}+\sqrt{5}$
2)a)$B=(1+\frac{x+\sqrt{x}}{1+\sqrt{x}})(1+\frac{x-\sqrt{x}}{1-\sqrt{x}})$
=$(1+\frac{\sqrt{x}(\sqrt{x}+1)}{\sqrt{x}+1})(1-\frac{\sqrt{x}(\sqrt{x}-1)}{\sqrt{x}-1})$
=$(1+\sqrt{x})(1-\sqrt{x})=1-x$
b)Khi $x=\frac{1}{1+\sqrt{2}}$
⇒$B=1-\frac{1}{1+\sqrt{2}}=1+\sqrt{2}-1=\sqrt{2}$
