I want to prove that Q(√p1+√p2)=Q(√p1,√p2) for p1,p2 primes. I know this was proved before for the generalized case of p1,....,pn primes here:
How to prove that Q[√p1,√p2,…,√pn]=Q[√p1+√p2+⋯+√pn], for pi prime?
And every proof of this kind of equality involves Galois theory but I still don't know Galois theory and I'm supposed to prove this with only basic Field theory(field extensions, irreducible polynomials, algebraic extensions, etc).
By definition Q(√p1+√p2) is the smallest field containing Q and √p1+√p2, also Q(√p1,√p2) is the smallest field containing √p1 and √p2. Or
Q(√p1,√p2)={a+b√p1+c√p2+d√p1p2∣a,b,c,d∈Q}
Q(√p1+√p2)={a+b(√p1+√p2)∣a,b∈Q}
Also I know that [Q(√p1,√p2):Q]=4 from this:
Proving that (Q[√p1,…,√pn]:Q)=2n for distinct primes pi.
Still don't know how to proceed proving this two extensions are the same.
Answer
Hint
Step 1 :
√p1−√p2=p1−p2√p1+√p2∈Q(√p1+√p2)
Step 2 :
√p1=(√p1+√p2)+(√p1−√p2)2∈Q(√p1+√p2).
I let you manage to show that √p2∈Q(√p1+√p2) as well and conclude the equality.
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