%符號表達式的符號替換
syms a b c d x
s=eig([a b;c d])%計算特征值
subexpr(s,x)%用x替換子表達式
f=sym('(x+y)^2+3*(x+y)+5')  %創建符號表達式
f1=subs(f)  %用工作空間的給定值替換x
f2=subs(f,'x+y','s')  %用s替換x+y
f3=subs(f,'x+y',5)  %用常數5替換x+y
f4=subs(f,'x','z')  %用z替換x