f := (x,y) -> arccos(x*y); v(u(x)) = x: diff(v(u(x)),x) = 1; ; Diff(arccos(y),y) = 1/(-sin(arccos(y))); diff(arccos(y),y); D[1](f)(0.5,-0.5); fy := D[2](f); fy(0.5,-0.5);