Final answer:
To find df using the chain rule and direct substitution, differentiate x^2 + y^2 with respect to x and y, and then substitute y = t^2. The resulting expression is 2x + 4t^3.
Step-by-step explanation:
To find df using the chain rule and direct substitution, we first find the partial derivative of f with respect to x, and the partial derivative of f with respect to y. Using the chain rule, we differentiate x^2 + y^2 with respect to x and y, and then substitute y = t^2. This gives us:
df/dt = (df/dx)(dx/dt) + (df/dy)(dy/dt)
df/dt = 2x(1) + 2y(2t)
df/dt = 2x + 4yt
Substituting y = t^2 into the expression, we get:
df/dt = 2x + 4t^3