Giải dùm e câu 1 với câu 4
View attachment 68636
a)[tex]\frac{1}{\sqrt{1}+\sqrt{2}}+\frac{1}{\sqrt{2}+\sqrt{3}}+...+\frac{1}{\sqrt{2004}+\sqrt{2005}}=\frac{\sqrt{2}-\sqrt{1}}{2-1}+\frac{\sqrt{3}-\sqrt{2}}{3-2}+\frac{\sqrt{4}-\sqrt{3}}{4-3}+...+\frac{\sqrt{2004}-\sqrt{2003}}{2004-2003}=\sqrt{2004}-1[/tex]
b)[tex]\Delta =[/tex] [tex](b^{2}+c^{2}-a^{2})^{2}-4.b^{2}.c^{2}=(b^{2}+c^{2}-a^{2}-2bc)(b^{2}+c^{2}+2bc+a^{2})=[(b-c)^{2}-a^{2}][(b+c)^{2}+a^{2}]< [(b-c)^{2}-(b+c)^{2}][(b+c)^{2}+a^{2}]< 0[/tex]