00:01
In this question, we are told that z equals to f of x plus 2y, and we are asked to show this identity.
00:09
First of all, we will introduce some notation.
00:13
We are going to call you x plus 2y.
00:22
And then in the formula for z, we will replace x plus 2y by you, so we are going to get that z equals to f of u.
00:30
And now we will use the chain rule to calculate d z over d x and d z over d y.
00:37
In other words, we have a function which depends on u and u in turn depends on x and y.
00:46
So basically this is f of u of x y.
00:52
By the chain rule d z over d x equals to d f over d x right that in turn equals to df over d u times d u over d x and sorry this should be a straight d here because f depends only on one variable you use a curved d when your function depends on two variables like you you depends on x and y right because it's x plus 2 y so we don't know what is d f over du but we can calculate d u over d x right it, du over d x from this formula equals to 1.
01:52
So that's going to be df over d u times 1, which is simply df over d .f over d .u...