Chapter 14.5, Problem 76E

Parabolic trajectory In Example 7 it was shown that for the parabolic trajectory r(t) = 〈t, t²〉, a = 〈0, 2〉 and $a=\frac{2}{\sqrt{1+4t^2}}(N+2tT)$. Show that the second expression for a reduces to the first expression.