[填空题]
Tom uses a compass tox )k0,i cay6nd draw a circle. The arm of the compass/ cf:w rw7ryzra3f 1t/ with the needle is 7 cm long, and the arm with the pencil is 8 cm long. The antfrwr3/7/zy :w1 ca frgle 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
