please answer just 8.1 and 8.2

8.1 Examine the function f for continuity in the point (0, 0). po . - yo for x # y f (x, y) = x- y 57-4 for x = y 8.2 Describe and sketch the contour lines z = f(x, y) the function f for z = a, z = b and z = C. 12xy (a) f(x, y) = 12 + 912) a = 0, 6=1, c= -1 (b) f(x, y) = ex-4y', a = 1, b = e, c= 8.3 Determine all partial first-order derivatives for the functions f : R2 -> R or. 9 : (0, 00 )2 - R (a) f(x, y) = exp (x3 + 2x + 1) . sin(xy), ( b) g(x, y) = x . In (x2 +y) +y3 . (cosh (# ) + sink (# ) ) . (cook (;) - sink (#) 8.4 Determine the local extreme points and saddle points of the function (a) f(x, y) = x2 - xy+ y3+ y (b) f (x,y) = xy - y2 -23 -22 (c) f(x, y) = 292 +ay -22 -y3