For what value of c will the circle with equation x^2 - 10x + y^2 + 4y + c = 0 have a radius of length 1?
+37146 Re-arrange to
x^2 - 10x + y^2 + 4y = -c now complete the squares for x and y
(x-5)^2 + (y+2)^2 = - c + 25 + 4 The red portion = r2
-c + 25 + 4 = 12
29 = 1 + c
28 = c
