[tex]A=\frac{1}{\sqrt{7}.(\frac{1}{\sqrt{13}}+\frac{1}{\sqrt{19}}+\frac{1}{\sqrt{7}})}+\frac{1}{\sqrt{13}.(\frac{1}{\sqrt{7}}+\frac{1}{\sqrt{19}}+\frac{1}{\sqrt{13}})}+\frac{1}{\sqrt{19}.(\frac{1}{\sqrt{7}}+\frac{1}{\sqrt{13}}+\frac{1}{\sqrt{19}})}[/tex]
[tex]A=\frac{1}{\frac{1}{\sqrt{7}}+\frac{1}{\sqrt{13}}+\frac{1}{\sqrt{19}}}.(\frac{1}{\sqrt{7}}+\frac{1}{\sqrt{13}}+\frac{1}{\sqrt{19}})=1[/tex]