fa(x)= 3/8 x2 * ln (x/a2) - 3/16 x2
f ´( x ) = 3/8 * ( 2 * x * ln ( x/a^2 ) + x^2 * 1/ x ) - 3 / 16 * 2 * x
f ´( x ) = 3/8 * ( 2 * x * ln ( x/a^2 ) + x ) - 3 / 8 * x | 3 / 8 ausklammern
f ´( x ) = 3/8 * ( 2 * x * ln ( x/a^2 ) + x - x )
f ´( x ) = 3/8 * ( 2 * x * ln ( x/a^2 ) )
f ´( x ) = 3/4 * x * ln ( x/a^2 )