find cos (x/2) when sint =12/13
sint = 12/13
then cost = 5/13 , by Pythagoras.
you probably meant find cos (t/2)
using cos 2A = 2cos^2 A - 1
cost = 2cos^2 (t/2) - 1
18/13 = 2cos^2 (t/2)
9/13 = cos^2 (t/2)
cos (t/2) = 3/sqrt(13)
then cost = 5/13 , by Pythagoras.
you probably meant find cos (t/2)
using cos 2A = 2cos^2 A - 1
cost = 2cos^2 (t/2) - 1
18/13 = 2cos^2 (t/2)
9/13 = cos^2 (t/2)
cos (t/2) = 3/sqrt(13)