Evaluate the Laplacian of the multi-variable function at the point ( 2 π , 3 )
f ( x , y ) = y e i x + y ln ( i x ) + x 2
Details and Assumptions :
i = − 1
Laplacian of a scalar function : ∇ 2 f ( x , y ) = ∂ x 2 ∂ 2 f + ∂ y 2 ∂ 2 f
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First we evaluate the second partial derivative of the function with respect to x . Then we evaluate the second partial derivative of the function with respect to y . Executing we find that
∂ x 2 ∂ 2 f = − y e i x − x 2 y + 2
and
∂ y 2 ∂ 2 f = 0
Once we have found our partial derivatives, we then find the linear combination of the two - per the definition of the Laplacian of a scalar function. Now we have that
∇ 2 f ( x , y ) = ∂ x 2 ∂ 2 f + ∂ y 2 ∂ 2 f = 2 − ( y e i x + x 2 y )
and finally, using Euler's Identity e i x = i sin x + cos x , we find the answer to the question:
∇ 2 f ( 2 π , 3 ) = 2 − ( 3 i sin ( 2 π ) + 3 cos ( 2 π ) + π 2 1 2 ) = 2 − ( 3 i + π 2 1 2 )