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**1. Homework Statement**

Hi, this is not homework exactly, I'm doing some exercises as part of my personal study. I'm analizing linear invariant systems and I'm stuck in an apparently trivial step, please, help. I have these integrals:

**2. Homework Equations**

integral( x(tau)*dtau, from -infinity to t) - integral ( x(tau)*dtau, from -infinity to t-T). I must prove that the former expression is the same as integral( x(t)-x(t-T), from -infinity to t).

**3. The Attempt at a Solution**

I have manipulated the expression a little, tried the change of variables s=tau+T; ds=dtau and now I have -integral(x(s-T)ds, from -infinity to t) + integral(x(tau)dtau, -infinity to t) but I don't know how continue. I'm pretty sure it's really simple, but there's something I'm missing or something that I don't fully understand about dummy variables and so. Help, please.