b(t) = 7,5 mg/cm3 * (e-t/0.5h)
( e^term ) = e^term * ( terrm ) ´ 
Falls
term = -t / ( 0.5 * h )
ist dann
term = 1 / ( 0.5 * h  ) * ( - t ) 
Ableitung nach t
term ´ =  1 / ( 0.5 * h  ) * ( - 1 )
term ´ =  - 1 / ( 0.5 * h  ) 
b(t) = 7,5 mg/cm3 * (e-t/0.5h)
b ´ ( t ) = 7,5 mg/cm3 * (e-t/0.5h) *  [ - 1 / ( 0.5 * h  ) ]