sinx = 3/5 = opposite side/hypotenuse
cosx = 4/5 = adjacent side/hypotenuse
5^2 = 4^2 + 3^2
25 = 16 + 9
sin(x/2) = + or - sqr(1-cosx)/2 = + sqr(1-4/5)/2 = - sqr(1/5/2) = sqr(1/10), ignore the negative root as x is in the 1st quadrant, and so is x/2.
sin(x/2) = 1/sqr10 = sqr10/10 = about 3.16/10 = 0.316