Riktiningsderivata; förklara lösning

$f(x,y)=\ln(1+2x+y)$

$\displaystyle \nabla f=\left(\frac{2}{2 x+y+1},\frac{1}{2 x+y+1}\right)$

$\nabla f_{(-1,2)}=(2,1)$

$f_v(-1,2)=\nabla f_{(-1,2)} \cdot v=(2,1)\cdot(\frac12,-\frac{\sqrt3}2)=1-\frac{\sqrt3}2$

Tack!