Answer:
δw/δr = 4π
δw/δθ = -8π
Step-by-step explanation:
Given the following functions
w = xy + yz + zx, x = r cosθ y = r sinθ, z = rθ where r = 4 and θ = π/2
We are to find δw/δr and δw/δθ
δw/δr = δw/δx•δx/δr + δw/δy•δy/δr + δw/δz•δz/δr
δw/δx = y+z
δx/δr = cosθ
δw/δy = x+z
δy/δr = sinθ
δw/δz = y+x
δz/δr = θ
Substitute the given values into the formula
δw/δr = (y+z)cosθ+(x+z)sinθ+(y+x)θ
Substitute the value of x, y and z in terms of theta into the resulting function
δw/δr = (y+z)cosθ+(x+z)sinθ+(y+x)θ
δw/δr = (rsinθ+rθ)cosθ+(rcosθ+rθ)sinθ+(rsinθ+rcosθ)θ
Substitute r = 4 and θ = π/2
δw/δr = (4sinπ/2+4π/2)cosπ/2+(4cosπ/2+4π/2)sinπ/2+(4sinπ/2+4cosπ/2)π/2
Note that cos π/2 = 0 and sinπ/2 = 1
δw/δr = (4+2π)(0)+(0+2π)(1)+(4(1)+4(0))π/2
δw/δr = 0+2π+4π/2
δw/δr = 2π+2π
δw/δr = 4π
For δw/δθ
δw/δθ = δw/δx•δx/δθ + δw/δy•δy/δθ + δw/δz•δz/δθ
δw/δx = y+z
δx/δθ = -rsinθ
δw/δy = x+z
δy/δθ =rcosθ
δw/δz = y+x
δz/δθ = r
Substitute the given values into the formula
δw/δθ = (y+z)-rsinθ+(x+z)rcosθ+(y+x)r
Substitute the value of x, y and z in terms of theta into the resulting function
δw/δθ = (y+z)-rsinθ+(x+z)rcosθ+(y+x)r
δw/δθ = (rsinθ+rθ)-rsinθ+(rcosθ+rθ)rcosθ+(rsinθ+rcosθ)r
Substitute r = 4 and θ = π/2
δw/δθ = (4sinπ/2+4π/2)-4sinπ/2+(4cosπ/2+4π/2)4cosπ/2+(4sinπ/2+4cosπ/2)(4)
Note that cos π/2 = 0 and sinπ/2 = 1
δw/δθ = (4+2π)(-4)+(0+2π)(0)+(4(1)+4(0))(4)
δw/δθ = -16-8π+0+4(4)
δw/δθ = -16+16-8π
δw/δθ = -8π
