Find a unit normal to the surface cos(xy)=e^z-2 at (1,pi,0)

## Answer

In this problem i have used the fact that < a b c> is the normal vector to the plane ax + by + cz = d.

cos(xy) = e^z - 2

--> z = ln(2+cos(xy))

now, equation of tangent plane to z = f(x,y) at point (xo,yo,zo) is given by:

z-zo = fx(xo,yo)(x-xo) + fy(xo,yo)(y-yo)

here, fx = -y sin(xy) / (2+cos(xy), fy = -x sin(xy) / (2+cos(xy)

at (x,y) = (1,pi): fx = 0, fy = 0,

equation of tangent plane is:

z -0 = 0

--> z = 0 ...(that is whole xy plane, and unit normal to xy plane is k(cap)

other way:

z = 0

--> 0x + 0y + 1z = 0

that is whole xy plane, and unit normal to xy plane is k(cap),

vector form is : < 0 0 1>

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