Question 375189
{{{cos3A = cos(2A + A) = cos2AcosA - sin2AsinA }}}
= {{{(2(cosA)^2-1)cosA - 2sinAcosAsinA = 2(cosA)^3-cosA - 2(sinA)^2cosA}}}
 ={{{ 2(cosA)^3-cosA -2(1-(cosA)^2)cosA = 4(cosA)^3 - 3cosA}}}.
Then {{{cos3A = 4(3/5)^3 - 3*(3/5)= -0.936}}}