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**Ares_D1** Having a little trouble with this problem. I keep getting zero as an answer which is incorrect. I know where I'm making the mistake, just don't know how to fix it.

**Problem:**

Compute the directional derivative for $\displaystyle f(x,y,z) = zexp(-2xy)$ at point $\displaystyle (1,1,-1), t = (1i,-3j,2k)$.

**Formula:**

$\displaystyle \frac{df}{ds} = t \cdot (\frac{\partial f}{dx}i, \frac{\partial f}{dy}j, \frac{\partial f}{dz}k)$.

Basically I'm getting stuck with the partial derivatives. I seems to me that the partial of either x, y, or z will give 0. No?