sai câu b,c nhé

\(a,\left(\dfrac{\sqrt{14}-\sqrt{7}}{1-\sqrt{2}}+\dfrac{\sqrt{15}-\sqrt{5}}{1-\sqrt{3}}\right):\dfrac{1}{\sqrt{7}-\sqrt{5}}\\ =\left(\dfrac{\sqrt{7}\left(\sqrt{2}-1\right)}{-\left(\sqrt{2}-1\right)}+\dfrac{\sqrt{5}\left(\sqrt{3}-1\right)}{-1}\right).\left(\sqrt{7}-\sqrt{5}\right)\\ =\left(-\sqrt{7}-\sqrt{5}\right).\left(\sqrt{7}-\sqrt{5}\right)\\ =-\left(\sqrt{7}+\sqrt{5}\right).\left(\sqrt{7}-\sqrt{5}\right)\\ =-\left(7-5\right)\\ =-2\)

\(b,\dfrac{2}{\sqrt{3}+1}-\dfrac{1}{\sqrt{3}-2}+\dfrac{12}{\sqrt{3}+3}\\ =\dfrac{2\left(\sqrt{3}-1\right)}{\left(\sqrt{3}+1\right)\left(\sqrt{3}-1\right)}-\dfrac{\sqrt{3}+2}{\left(\sqrt{3}-2\right)\left(\sqrt{3}+2\right)}+\dfrac{12\left(\sqrt{3}-3\right)}{\left(\sqrt{3}+3\right)\left(\sqrt{3}-3\right)}\\ =\dfrac{2\sqrt{3}-2}{3-1}-\dfrac{\sqrt{3}+2}{3-4}+\dfrac{12\sqrt{3}-36}{3-9}\\ =\dfrac{2\left(\sqrt{3}-1\right)}{2}-\dfrac{\sqrt{3}+2}{-1}+\dfrac{6\left(2\sqrt{3}-6\right)}{-6}\\ =\sqrt{3}-1+\sqrt{3}+2-2\sqrt{3}+6\\ =7\)

\(\left(x+1\right)\left(x+3\right)>\left(x+2\right)\left(x+4\right)\\ \Leftrightarrow x^2+x+3x+3>x^2+2x+4x+8\\ \Leftrightarrow x^2+4x+3>x^2+6x+8\\ \Leftrightarrow x^2+6x+8-x^2-4x-3< 0\\ \Leftrightarrow2x+5< 0\\ \Leftrightarrow x< -\dfrac{5}{2}\)

Tìm `x in Z` để `x^4 +5x^2 +x+2` là số chình phương

Tìm `x,y,z` nguyên dương sao cho `3^x + 2^y = 1+2^z`

Cho `x,y in Z` thỏa mãn: `3(x^2 -1)=2(y^2 -1)`

CMR: `x^2 -y^2 vdots 40`

\(\left(3+\sqrt{5}\right)\sqrt{\left(\sqrt{5}-3\right)^2}\\ =\left(3+\sqrt{5}\right)\left|\sqrt{5}-3\right|\\ =\left(3+\sqrt{5}\right)\left(3-\sqrt{5}\right)\\ =3^2-\sqrt{5^2}\\ =9-5\\ =4\)

\(\sqrt{80}-\sqrt{\left(2-\sqrt{5}\right)^2}+\sqrt{\dfrac{16}{5}}\\ =4\sqrt{5}-\left|2-\sqrt{5}\right|+\dfrac{4}{\sqrt{5}}\\ =4\sqrt{5}-\left(\sqrt{5}-2\right)+\dfrac{4\sqrt{5}}{5}\\ =4\sqrt{5}-\sqrt{5}+2+\dfrac{4\sqrt{5}}{5}\\ =3\sqrt{5}+2+\dfrac{4\sqrt{5}}{5}\\ =\dfrac{15\sqrt{5}}{5}+\dfrac{10}{5}+\dfrac{4\sqrt{5}}{5}\\ =\dfrac{15\sqrt{5}+10+4\sqrt{5}}{5}\\ =\dfrac{10+19\sqrt{5}}{5}\)

có đk j đâu