f(x,y)=3xy^2-x-y

a)find the equation of the tangent plane to the graph of f(x,y,) at the point (2,1,3)
b) use linear approximation to estimate the value off f(2.1,0.9)
c) find the directional derivative of f at (2,1) in the direction of î+j

∂f/∂x = 3y^2 - 1

∂f/∂y = 6xy - 1
so the tangent plane at (2,1,3) is
z-3 = 2(x-2) + 11(y-1)

(b) ∆z ≈ 2(0.1) + 11(-0.1)
so f(2.1,0.9) = 3+0.2-1.1 = 2.1

(c) the direction cosines are 1/√2,1/√2
so the directional derivative is
2/√2 + 11/√2 = 6√2