deletions | additions       diff --git a/From_3_vec_a_t__.tex b/From_3_vec_a_t__.tex  index ff4079e..c362b1b 100644  --- a/From_3_vec_a_t__.tex  +++ b/From_3_vec_a_t__.tex 
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$\Delta$:  From (3):  $$\vec a(t)\Delta t^2=\vec{x}(t+\Delta t) - 2\vec x(t) + \vec x(t-\Delta t)$$