Question

A circle is graphed in the coordinate plane with the equation x2 + y2 = 81. At what points does the graph intersect the y-axis?

Answer 1

Words to the wise:  Please express exponentials as x^2, not x2.

x2 + y2 = 81  should be   x^2 + y^2 = 81  = 9^2
Comparing this to             x^2 + y^2 = r^2, we see that r= radius=9.  y-intercepts are where x=0, so purposely set x=0 and solve for y:

0^2 + y^2 = 9^2.  Then y = 9 or y = -9.

The graph intersects the y-axis at (0,9) and (0, -9).