MATLAB: Error index exceeds matrix dimension. I am not using matrix

MATLABpartial derivative

syms th d alpha a th1 th2 l1 l2 lc1 lc2 I1z I2z m1 m2
d(1,1)=m1*lc1^2+m2*(l1^2+lc2^2+2*lc2*l1*cos(th2))+I1z+I2z;
d(1,2)=m2*(lc2^2+l1*lc2*cos(th2))+I2z;
diff(d(1,2),th(1))+diff(d(1,1),th(2))
Index exceeds matrix dimensions.

Best Answer

   syms th d alpha a th1 th2 l1 l2 lc1 lc2 I1z I2z m1 m2
  d(1,1)=m1*lc1^2+m2*(l1^2+lc2^2+2*lc2*l1*cos(th2))+I1z+I2z;
  d(1,2)=m2*(lc2^2+l1*lc2*cos(th2))+I2z;
  diff(d(1,2),th1)+diff(d(1,1),th2)
It should be th1 and th2 not th(1), th(2)
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