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http://higheredbcs.wiley.com/legacy/college/nise/0471794759/appendices/app_i.pdf

I am looking at how they obtained (I.24) from (I.25) on page 3 and 4.

Firstly we have:

[itex]e^{-\textbf{A}t}x(t)-x(0)=\int{e^{-\textbf{A}t}\textbf{Bu}(\tau)d\tau}[/itex]

Then this is derived to:

[itex]x(t)=e^{-\textbf{A}t}x(0)+\int{e^{-\textbf{A}(t-\tau)}\textbf{Bu}(\tau)d\tau}[/itex]

my question is why does the derived equation have [itex]e^{-\textbf{A}t}[/itex]. I thought it would have been [itex]e^{\textbf{A}t}[/itex] instead. Can someone explain this too me. The text didnt help me.