Răspuns:
k = [tex]\frac{83}{700}[/tex], k = 0.11857...
Explicație pas cu pas:
[tex]\frac{1}{4k} +\frac{1}{5k} +\frac{1}{7k} =5\\\\\frac{35}{140k} +\frac{28}{140k} +\frac{20}{140k} =5\\\\\frac{35+28+20}{140k} =5\\[/tex]
[tex]\frac{35+48}{140k} =5\\\\\frac{83}{140k}=5\\\\k=\frac{83}{140*5} \\[/tex]
[tex]k=\frac{83}{700} \\\\k=0.11857[/tex]
Succes! :)
