1. The problem statement, all variables and given/known data In a certain region, the electric potential due to a charge distribution is given by the equation V (x, y, z) = (3x2y2+yz3-2z3x)V0/a4 where a, x, y, and z are measured in meters and V and V0 are in volts. What is the magnitude of the electric field at the position (x, y, z) = (a, a, a)? 2. Relevant equations E=-(dV/dx(i)+dV/dy(j)+dV/dz(k)) 3. The attempt at a solution Taking the negative derivative of V(x,y,z) and inputing "a" gives E=-V0/a(4,7,-3) I would have thought that this was the complete solution, but the solution that goes on to absval(E)=V0/a(42+72+32)½ =V0/a(74)½ I haven't taken multi-variable calculus yet (it wasn't a requirement for the course), so I'm a little confused as to what's going on in the part where the absolute value of E is equal to V0/a4(2+72+32)½. Could someone explain to me how this works, specifically, why should I take the abs value of E, and add up the squares of the derivatives and then take their square root to get the answer? Thanks!