For each of the following functions f(x,y) calculate f(0,0) first. Then, use Taylor expansion up to the second order to estimate f(1,1). How close are the estimates to the real f(1,1)'s?
f(x,y)= x^2+3xy+y^2
f(x,y)=e^(-x^2-y^2)
f(x,y)=1/(x+y-2)
f(x,y)=sinxy