[填空题]
Tom uses a compass to draw a z, 9 e4+qsulpyn o,nzqm;.l 7ncircle. The amc3b6 5f* mu uyn6xr.frm of the compass with the needle is 7 cm long, and the arm with the pencil is 8*cxfr6ufy6b. 3 m 5nmu 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
