There is no specific instruction. So let's analyze the function f(x,y)=excosy. Partial derivative with respect to x:
∂x∂f=excosy Partial derivative with respect to y:
∂y∂f=−exsiny Second partial derivative with respect to x:
∂x2∂2f=excosy Second partial derivative with respect to y:
∂y2∂2f=−excosy Mixed partial derivative:
∂x∂y∂2f=−exsiny ∂y∂x∂2f=−exsiny Laplacian:
∇2f=∂x2∂2f+∂y2∂2f=excosy−excosy=0 The function is harmonic.