In Example 7, change the + before 6y to −.

Data from Example 7

Display the graph of the circle 3x 2 + 3y 2 + 6y − 20 = 0 on a calculator. To fit the form of a quadratic equation in y, we write 3y² + 6y + (3x² − 20) = 0. Now, using the quadratic formula to solve for y, we let

which means we get the two functions

which are entered into the calculator to get the view shown in Fig. 21.38.

The window values were chosen so that the length along the x-axis is about 1.5 times that along the y-axis so as to have less distortion in the circle. The gaps shown on the left and right sides of the circle should not be there, but graphing utilities sometimes have difficulty graphing parts of curves that are nearly vertical.

a = 3 b = 6 -6 6 y = c = 3x - - 20 4(3)(3x - 20) 2(3)