- #1

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## Homework Statement

I am a bit new to Einstein notation. Why is this true:

[tex] \eta_{\mu \nu} \frac{d \delta x ^ {\mu}}{d \tau} \frac{d x ^ {\nu}}{d \tau} = \frac{d \delta x ^ {\mu}}{d \tau} \frac{d x _ {\nu}}{d \tau} [/tex]

How can you go from 16 terms on the LHS to just 4 terms on the RHS. If it is relevant this is a variational calculus problem and x^mu is the spacetime four-vector and delta is the variation and tau is some parametrization.

EDIT: this equation is wrong. see below

## Homework Equations

## The Attempt at a Solution

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