设圆心角θ,半径r
c=2πr*θ/2π+2r=r(θ+2)=20
S=πr^2*θ/2π=r^2*θ/2=16
r^2*(20/r -2)/2=16
20r-2r^2=32
r^2-10r+16=0
(r-2)(r-8)=0
r=2或r=8
r=2 θ=8>2π舍去
r=8cmr=2可以取吗舍弃了，看过程