[填空题]
Tom uses a compass to draw a rc2 0rhllgu;ykwl ; 8fk 5*,pfcircle. The 8 6/vyoqt+( fy hdv7qrv92)bo(f gcmparm of the compass with the needle is 7 cm long, and the a/+vp6vo2fg7 r)f9 oc vqy bd8 (thmyq(rm with the pencil is 8 cm long. The angle between the arms is θ as shown in the diagram below.
1.Find an expression for the radius of the circle to be drawn in terms of θ.
r=$\sqrt{a-b cosθ}$, a= , b= .
2.Hence, find the rate of change in r with respect to θ when θ=30$^∘$ .
the rate of change is cm per radian
