i want to define field \mathbb Q (\sqrt 2,\sqrt 3) what command should use?

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You can do the following:

R.<x> = PolynomialRing(QQ)
f = (x^2-2)*(x^2-3)
K.<a> = f.splitting_field()

Note that f.splitting_field() requires a name for the primitive element of the field (a here), which is passed here by using the shorthand notation, as shown in the documentation.

To identify the elements of the splitting field K which correspond to the roots of f you can do e.g.

f.change_ring(K).roots()

or in this case also something like

sqrt(K(3))

The most natural way is probably as follows.

sage: K = QQ[sqrt(2), sqrt(3)]
sage: K.inject_variables()
Defining sqrt2, sqrt3
sage: (sqrt2 + sqrt3)^2
2*sqrt3*sqrt2 + 5

The case of adjoining two algebraic numbers with the same minimal polynomial is more involved, see:

• Ask Sage question #40389: Extension field adjoining two roots

• Math Stack Exchange question 2586636: Extension field adjoining two roots in Sage

Alternatively, you can do:

sage: number_field_elements_from_algebraics([QQbar(2).sqrt(), QQbar(3).sqrt()],minimal=True, same_field=True)
(Number Field in a with defining polynomial y^4 - 4*y^2 + 1,
 [-a^3 + 3*a, -a^2 + 2],
 Ring morphism:
   From: Number Field in a with defining polynomial y^4 - 4*y^2 + 1
   To:   Algebraic Field
   Defn: a |--> 0.5176380902050415?)