Answer:
1. r=17
2. (-15,14) or (-15,-16)
Step-by-step explanation:
The radius of the circle is the distance from the center to the point on the circle, thus
[tex]r=\sqrt{(8-(-7))^2+(7-(-1))^2}=\sqrt{15^2+8^2}=\sqrt{225+64}=\sqrt{289}=17.[/tex]
The equation of the circle is
[tex](x-(-7))^2+(y-(-1))^2=r^2\\ \\(x+7)^2+(y+1)^2=289.[/tex]
If point lies on this circle, then its coordinates satisfy the circle's equation:
[tex](-15+7)^2+(y+1)^2=289\\ \\64+(y+1)^2=289\\ \\(y+1)^2=225\\ \\y+1=15\text{ or }y+1=-15\\ \\y=14\text{ or }y=-16[/tex]
