When … the body should travel in a straight ____

When $F=0$ the body should travel in a straight ____.
The equation $q_{\theta \theta }+q=0$ allows $q=\cos \theta$, in which case the path $1/r=\cos \theta$ is ____. Extra credit: Mark off equal distances on a line, connect them to the sun, and explain why the triangles have equal area. So $dA/dt$ is still constant.

Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §12.4 Polar Coordinates and Planetary Motion, #29