The obvious way: construct an iosomorphism from A to (B intersection C).
Of course, how you construct such an isomorphism will depend on exactly what A, B, and C are, since whether they are isomorphic depends on what they are!
That is, find a function that assigns, to every member of A, a specific member of B intersect C and then show that it:
1) preserves the operation: f(x*y)= f(x)*f(y)
2) maps the indentity in A to the identity in B.
3) is one-to-one.
4) maps A "onto" B intersect C.
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