Step-by-step explanation:
Given the function
f(x, y) = 5 + x ln(xy − 9) at the point (5, 2)
For the function to be differentiable at the given point, the differential of the function with respect to x and y must be continuous at the point.
Fx is the differential of the function with respect to x.
Using product rule to get f(x):
fx = 0 + x(1/(xy-9)y) + ln(xy-9)
fx = xy(1/(xy-9)) + ln(xy-9)
Substitute point (5, 2) into fx
fx = 5(2)(1/5(2)-9) + ln(5(2)-9)
fx = 10(1/(10-9)) + ln(10-9)
fx = 10(1/1) + ln1
fx = 10 + 0
fx = 10
For fy:
fy = 0 + x²(1/(xy-9))+ 0
fy = x²(1/(xy-9))
Substitute point (5, 2) into fy
fy = 5²(1/(5(2)-9))
fy = 25(1/10-9)
fy = 25(1/1)
fy = 25
Next is to calculate f(5,2)
f(5,2) = 5 + 5 ln(2(5) − 9)
f(5,2) = 5 + 5 ln(10− 9)
f(5,2) = 5 + 5 ln1
f(5,2) = 5 + 5(0)
f(5,2) = 5
Since fx ≠ fy, hence the function is not differentiable at the given point
