Referencing the Component of a vector

Question

If I were to define a vector

r = {2, 0} + t*({5, 4} - {2, 0})
>>{2 + 3 t, 4 t}
Sqrt[(D[r[1]])^2 + (D[r[2]])^2]

How do I properly reference a single component of the vector so that I can use it for something else like derive. This is for program to solve line integrals.

Answer 1

Sqrt[(D[r[1]])^2 + (D[r[2]])^2]

You can't just write D[somthing] you need to write D[something, variable].

You can try

r = {2, 0} + t*({5, 4} - {2, 0});

Sqrt[ D[ r[[1]], t]^2 + D[ r[[2]], t]^2]
(*5*)

If it makes it easier, you could give the components names, and just use the names

x = r[[1]];
y = r[[2]];

Sqrt[ D[x,t]^2 + D[y,t]^2 ]
(*5*)

Or You could also write

Norm@Grad[r, {t}] 
(*5*)

which gives same result if I am following what you are trying to do right.