# Commutativity of Tor

Let $A$ be a commutative ring with $1$ and $M,N$ be $A$-modules. Can you give a quick proof that $\textrm{Tor}_i(M,N) \cong \textrm{Tor}_i(N, M)$ using derived categories?

In his Homological algebra book, Weibel proves this with an argument via a double complexes: the so-called "acyclic assembly lemma", and from what I understand this argument can be essentially reworded into the language of spectral sequences. Hartshorne's discussion (in "Residues and Duality") of derivatives of functors in two variables is quite short, but it's not clear to me if this result (commutativity of Tor) immediately follows from the relevant derived category formalism.

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Yes, the derived category of a commutative ring is symmetric monoidal $M\otimes_A^{\mathbb L}N=N\otimes_A^{\mathbb L}M$ and Tor is the homology of this tensor product.
To relate this to the usual definition of Tor (derived functors of $M\otimes -$ or $-\otimes N$), presumably you need to show that the derived tensor product $M\otimes^L N$ can be computed by resolving only one of the two factors. Doesn't that require some kind of double-complex type argument? –  Charles Rezk Apr 9 '13 at 14:22