出现subs无法替换符号的现象

 A
 
A =
 
((v - w)*(tanh((v/2 - w/2)*(t + (2*atanh((2*v*infect(may))/(v - w) - 1))/(v - w))) + 1))/(2*v)
 
>> B=subs(A,v,0.0710);
>> B
 
B =
 
((v - w)*(tanh((v/2 - w/2)*(t + (2*atanh((2*v*infect(may))/(v - w) - 1))/(v - w))) + 1))/(2*v)
 

matlab初学者提问，为何符号方程中的v没有被替换为数值？