Double derivating a function and after turn in the answer 2 constans

Hello!

I am stucked from 5-6 hours at this point:

The function is: [arctan(x−3yx+3y)] and the exercise is to calculate d ^ 2 f / d x ^ 2 and after that i shall put the values (1, 4*log3) instead x and y.

I tryed everything and i reached many hours ago here:

f = inline(atan((x-3*y)/(x+3*y)));
der = diff(f(x,y),x,x)

And the result is: - (2/(x + 3*y)^2 - (2*(x - 3*y))/(x + 3*y)^3)/((x - 3*y)^2/(x + 3*y)^2 + 1) - (((2*x - 6*y)/(x + 3*y)^2 - (2*(x - 3*y)^2)/(x + 3*y)^3)*(1/(x + 3*y) - (x - 3*y)/(x + 3*y)^2))/((x - 3*y)^2/(x + 3*y)^2 + 1)^2 which can be simplified by:

pretty(simplify(diff(f(x,y),x,x)))

And the result is: 6 x y

- -------------

2 2 2

(x + 9 y ) .

Now, i searched everywhere, i asked too many people and i didn.t got answer for my specific question: how can i replace at this stage x = 1 and y = 4*log(3) and make calculus with them in the format as you seen above?

Thank you very much!

P.s. I am very begginer, but the YouTube, Google, Courses at University didn.t helped me.

Answer 1

[Walter Roberson]

syms x y
f(x,y) = atan((x-3*y)/(x+3*y))

f(x, y) = 

der = diff(f(x,y),x,x)

der = 

sder = simplify(der)

sder = 

subs(sder, [x, y], [1, 4*log(sym(3))])

ans = 

[Dacian Andrei Firca]

Oh thank you very much!!

You are my hero!!