Explain why the function is differentiable at the given point. f(x, y) = 5 + x ln(xy − 9), (5, 2)

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