A = alpha , B = beta
Show that the linear approximation of the function f(x,y) = x^a y^B at (1,1) is
x^a Y^B = 1 + a(x-1) + B(y-1) .
Since f(x,y)=x^a y^B, then we have
df/dx=a*x^(a-1) y^B, ...
This shows how to write a linear approximation of a function at a point.