Problem 1 :
Find the partial derivatives of the following functions at the indicated points.
(1) f (x, y) = 3x2 − 2xy + y2 + 5x + 2, (2, −5)
(2) g(x, y) = 3x2 + y2 + 5x + 2, (1, −2)
(3) h(x, y, z) = x sin(xy) + z2x, (2, π/4, 1)
(4) G(x, y) = e(x+3y) log(x2+y2), (-1, 1)
Problem 2 :
For each of the following functions find the fx and fy and show that fxy = fyx
(1) f (x, y) = 3x/(y+sinx)
(2) f(x, y) = tan-1(x/y)
(3) f(x, y) = cos (x2-3xy)
Problem 3 :
If
U(x, y, z) = (x2+y2)/xy + 3z2y
find (∂u/∂x), (∂u/∂y) and (∂u/∂z)
Problem 4 :
If U(x, y, z) = log (x3+y3+z3), find
(∂u/∂x) + (∂u/∂y) + (∂u/∂z)
Problem 5 :
For each of the following functions find gxy, gxx, gyy,and gyx.
(i) g(x, y) = xey+3x2y
(ii) g(x, y) = log(5x+3y)
(iii) g(x, y) = x2+3xy-7y+cos(5x)
Problem 6 :
Let
w(x, y, z) = 1/√(x2+y2+z2), (x, y, z) ≠ (0, 0, 0).
Show that (∂2w/∂x2) + (∂2w/∂y2) + (∂2w/∂z2)
Problem 7 :
If V(x, y) = ex(x cos y - y sin y), then prove that
∂2w/∂x2 = ∂2w/∂y2
Problem 8 :
If w(x, y) = xy + sin(xy) , then prove that
∂2w/∂y∂x = ∂2w/∂x∂y
Problem 9 :
If V(x, y, z) = x3+y3+z3+3xyz, show that
∂2v/∂y∂z = ∂2v/∂z∂y
Problem 10 :
A firm produces two types of calculators each week, x number of type A and y number of type B . The weekly revenue and cost functions (in rupees) are
R(x, y) = 80x + 90y + 0.04xy − 0.05x2 − 0.05y2 and
C(x, y) = 8x + 6y + 2000 respectively.
(i) Find the profit function P(x, y) ,
(ii) Find ∂P/∂x (1200, 1800) and ∂P/∂y (1200, 1800) and interpret these results.
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