P.S.:
To elaborate a bit more, note the difference:
sage: a = 2/3
sage: R.<x>=RealField(100)[]
sage: R
Univariate Polynomial Ring in x over Real Field with 100 bits of precision
sage: type(x)
sage.rings.polynomial.polynomial_real_mpfr_dense.PolynomialRealDense
sage: type(a*x+a)
sage.rings.polynomial.polynomial_real_mpfr_dense.PolynomialRealDense
sage: x=var('x')
sage: type(x)
sage.symbolic.expression.Expression
sage: type(a*x+a)
sage.symbolic.expression.Expression