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Well, it is hard to decide which is a simplified version of the other. For many, the first expression is simpler than the second since the involved trigonometric functions are eveluated on x, not pi/6-x. But i agree that something might be missing to express what kind of modification the user expect from Sage.

However, Sage somehow knows that the two expressions are equal:

sage: bool((sqrt(3)/3*cos(x)+1/3*sin(x)) == 2/3*cos(pi/6-x))
True

See also:

sage: (sqrt(3)/3*cos(x)+1/3*sin(x)).full_simplify()
1/9*sqrt(3)*(sqrt(3)*sin(x) + 3*cos(x))
sage: ( 2/3*cos(pi/6-x)).full_simplify()
1/9*sqrt(3)*(sqrt(3)*sin(x) + 3*cos(x))