4) You have the following equation:
[tex]x^2+10x+y^2-16=0[/tex]In order to determine the radius and center of the circle, complete squares for x. You don't complete squares for y because there is no term with y in the given expression. It is only a y^2 term.
By adding 25 and subtracting 25 left side of the equation you obtain:
[tex]x^2+10x+25+y^2-16-25=0[/tex]The first three terms are a perfect square (x + 5)^2, then, by using this factor and by simplifying in the previous equation you can write:
[tex](x+5)^2+y^2-41=0[/tex]Finally, add 41 both sides:
[tex](x+5)^2+y^2=41[/tex]The previous equation is in standard form for a circle equation:
[tex](x-h)^2+(y-k)^2=r^2[/tex](h,k) is the center of the circle and r the radius. By comparin the previous equation with the expression you obtain you obtain:
center of the circle = (-5,0)
radius r = √41