```
using LinearAlgebra
using SymPy
@vars t x y
m=[cos(t) -sin(t) 0;
sin(t) cos(t) 0;
0 0 1];
a = inv(m)*[x;y;t]
b=a'*a
```

So, I want the coefficient of sin(t) or (sin(t))^2 in final expression b.

```
using LinearAlgebra
using SymPy
@vars t x y
m=[cos(t) -sin(t) 0;
sin(t) cos(t) 0;
0 0 1];
a = inv(m)*[x;y;t]
b=a'*a
```

So, I want the coefficient of sin(t) or (sin(t))^2 in final expression b.

If you use `b.expand().coef(sin(t))`

or `b.expand().coeff(sin(t)^2)`

, you should get corresponding coefficients.

```
@vars t
@vars x y
m=[cos(t) -sin(t) sin(t);
sin(t) cos(t)*cos(t) 0;
0 0 1];
a = inv(m)*[x;y;t]
b=simplify.(a.T*a)
b.expand().coeff(sin(t))
```

I got an error,

KeyError: key :coeff not found

one f in coef

1 Like

Your `b`

is a vector, not a scalar. Try b[1] and the above should flow through. (In the first post, you have ` b=a'*a`

which dispatches to `dot`

which should return a scalar in this usage.)

1 Like