Step by step
The question
Question: find dz/dt when z = xy², x = 3t and y = t².
1 Draw the tree
Draw a tree.
z depends on x and y.
Each of them depends on t.
Each branch has its own slope.
To go from z down to t, there are two paths.
Multiply along each path, then add the paths.
2 The pieces
Term by term.
In xy², y² is fixed and waits in front.
x gives 1.
So y².
So ∂z/∂x = y².
Term by term.
In xy², x is fixed and waits in front.
y² gives 2y.
So 2xy.
So ∂z/∂y = 2xy.
x = 3t, so dx/dt = 3.
y = t², so dy/dt = 2t.
3 Use the chain rule
The chain rule: the x path plus the y path.
Put the four pieces in.
Careful!
Many people use only one path.
z changes through x and through y, so both paths count.
4 Write it in t
The answer should be in t only.
So put x = 3t and y = t² in.
Each x becomes 3t and each y becomes t².
Multiply each part out: numbers with numbers, and add the powers of t.
Add the like terms: dz/dt = 15t⁴.
5 Check
Let's check another way.
Put x and y in first: z = 3t⁵.
Its derivative is 15t⁴.
The same answer.
Right!
Let's look back.
First: draw the tree.
Then: the pieces.
Then: use the chain rule.
Then: write it in t.
Last: check.