MATLAB隐函数画的图和数值求解的有差别

下文为隐函数画图的程序（图1）
clear
syms b c a y1 y2 t
b=1;
c=2;
a=2;
y1=0.5;
eqn=t/(t - y1/(y1 - 1))==-((a*t)/((a - 1)*(t - 1)) - ((b + c*((t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))/(t - 1) + 1))*(t - 1))/(y2*(c - ((b + c*((t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))/(t - 1) + 1))*(t - 1))/(t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1)))))*(t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))))/(((b + c*((t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))/(t - 1) + 1))*(t - 1))/(y2*(c - ((b + c*((t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))/(t - 1) + 1))*(t - 1))/(t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1)))))*(t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))) - 1)
ezplot(eqn)
axis([0.4 0.5 0 0.6])
xlabel('y2');ylabel('t');

数值求解：
clear
syms t b c a y1 y2
b=1;
c=2;
a=2;
y1=0.5;
y2=0.5;
eqn=t/(t - y1/(y1 - 1))==-((a*t)/((a - 1)*(t - 1)) - ((b + c*((t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))/(t - 1) + 1))*(t - 1))/(y2*(c - ((b + c*((t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))/(t - 1) + 1))*(t - 1))/(t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1)))))*(t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))))/(((b + c*((t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))/(t - 1) + 1))*(t - 1))/(y2*(c - ((b + c*((t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))/(t - 1) + 1))*(t - 1))/(t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1)))))*(t + (t*(t - 1))/((a - 1)*(t - y1/(y1 - 1))))) - 1)
solve(eqn,'t')  
结果分别为ans =1/3， -8.2749，   -0.7251。正解1/3很明显不是图中y2=0.5对应的t点。