Let X = X_1 / X_2, and A = X_1 / X_2. Using the exact sequence of triples, show that if the inclusion (X_1, A) --> (X, X_2) induces an isomorphism on homology, then the same holds for the inclusion (X_2, A) --> (X, X_1).
X_1 is X subscript 1
/ is union
/ is intersection
--> is an inclusion map
H_q (X, A) is the quotient module, the qth relative homology module of X mod A
Need a step-by-step proof outline.
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