L增加3分之1,也就变成3分之4L
L=3.14d
则新圆周长4L/3=3.14D,即L=3.14*3/4D
所以D=4d/3
(3.14D^2/4-3.14d^2/4)/(3.14d^2/4)
=(D^2-d^2)/d^2=(16d^2/9-d^2)/d^2=16/9-1=7/9
所以面积增加9分之7