$$ f_{a,b} (t)= a\cdot (t-b) \cdot e^{-(t-b)} $$
$$ f_{a,b} (t)= 0 $$
$$ 0= a $$
$$ 0= t-b $$
ehochirgendwas wird nie Null
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$$ f_{a,b} (t)= a\cdot (t-b) \cdot e^{-(t-b)} $$
$$ f'_{a,b} (t)= a\cdot e^{-(t-b)} +a\cdot (t-b) \cdot (-e^{-(t-b)})$$
$$ f'_{a,b} (t)= a\cdot \left(e^{-(t-b)} -(t-b) \cdot e^{-(t-b)}\right)$$
$$ f'_{a,b} (t)= a\cdot e^{-(t-b)} \left(1-(t-b) \right)$$
$$ f'_{a,b} (t)= a\cdot \left(1-t+b \right) \cdot e^{-(t-b)}$$
$$ f'_{a,b} (t)= 0$$
$$ 0= a\cdot \left(1-t+b \right) \cdot e^{-(t-b)}$$
$$ 0= a$$
$$ 0= 1-t+b $$