matlab解方程组问题怎么解都是空解求解答

[Yiming~]

R1=sym('R1')
R2=sym('R2')
R3=sym('R3')
R4=sym('R4')
R5=sym('R5')
R6=sym('R6')

syms R1 R2 R3 R4 R5 R6 I1 I2 I3 I4 I5 U U2
f1=sym('-20+I1*R5-I2*R1-U+R4*I3==0');
f2=sym('I2*R1+I5*R2-10==0');
f3=sym('U+I4*R3-I5*R2==0');
f4=sym('-I4*R3-I3*R4+3.5*R6-U2==0');
f5=sym('I1-I3-3.5==0');
f6=sym('I3-I4-2==0');
f7=sym('I5-I2-2==0');
[I1,I2,I3,I4,I5,U,U2]=solve([f1,f2,f3,f4,f5,f6,f7])
敬告是这么报的，
Support of character vectors that are not valid variable names or define a number will be removed in a future release. To create symbolic expressions, first create symbolic
variables and then use operations on them.