+26 Find the standard equation of the circle which satisfies the given conditions
center at (5,6), through (9,4)
center at (-2,3), tangent to the line y=8
center at (-2,3), tangent to the line x= -10
center in the third quadrant, tangent to both the x-axis and y-axis, radius 7
a diameter with endpoints (-9,2) and (15,12)
concentric with x^2+y^2+2x-4y=5, radius is 7
