怎么这个方程解不出来？
clc;clear all;
close all;
R=500;
h=200;
ra=100;
k=0.5;
delta0=pi/3;
mu=0.7;
syms rb
d0=(rb-ra)*tan(delta0);
OC3=sqrt(R^2+(d0+h)^2);
OC2=OC3-k*d0;
R1=sqrt(OC2^2-(d0+h)^2);
equ1=(mu*R1-rb)*((mu*R1)^2+d0^2)^(3/2)+ra*rb*mu*R1*d0;
simple(equ1);
equ2=simple(equ1)
solve('equ2=0','rb')
