Hi Alex,
From the documentation
FindMinimum[f,{x,Subscript[x, 0],Subscript[x, 1]}] searches for a
local minimum in f using Subscript[x, 0] and Subscript[x, 1] as the
first two values of x, avoiding the use of derivatives.
Not sure why you are using {t, tMax, K*100}
, K*100 = 100,000
which is way too large. This gives the right answer
FindMinimum[y3, {t, tMax + 0.1}]
(* {17.9542, {t -> 56.8603}} *)
or
FindMinimum[y3, {t, tMax, tMax + 1}]
If the solution has high frequency oscillations it is possible that FindMinimum
can miss the next minimum depending on the x0, x1
values used.