Thomas' Calculus 13th Edition

$a=\dfrac{4}{3} \ T + \dfrac{2 \sqrt 5}{3} \ N$
We calculate the velocity and acceleration as follows: $v(t)=\dfrac{dr}{dt}=i+2j+2tk \implies |v(t)|=\sqrt {1^2+(2)^2+(2t)^2}=\sqrt {4t^2+5}$ and $a(t)=\dfrac{d \ v(t)}{dt}= \dfrac{4t}{\sqrt {4t^2+5}}$ $|a(1)|=\dfrac{4(1)}{\sqrt {4 (1)^2+5}}=\dfrac{4}{3}$ Now, $a_{N}=\sqrt {|a|^2 -a^2_{T}}=\sqrt {2^2 -(\dfrac{4}{3})^2}=\dfrac{2 \sqrt 5}{3}$ So, $a=a_T T+a_{N}=\dfrac{4}{3} \ T + \dfrac{2 \sqrt 5}{3} \ N$