cos(alpha+beta)=cos(alpha)cos(beta)-sin(alpha)sin(beta)cos(α+β)=cos(α)cos(β)−sin(α)sin(β)
Making alpha=beta->cos(2alpha) = cos(alpha)^2-sin(alpha)^2α=β→cos(2α)=cos(α)2−sin(α)2
but cos(alpha)^2+sin(alpha)^2=1cos(α)2+sin(α)2=1 then
cos(alpha)^2=( 1+cos(2 alpha))/2cos(α)2=1+cos(2α)2
so int cos(x)^2dx = int( 1+cos(2 x))/2dx = 1/2int dx + 1/2intcos(2x)dx∫cos(x)2dx=∫1+cos(2x)2dx=12∫dx+12∫cos(2x)dx
Finally int cos(x)^2 dx = x/2 + 1/4 Sin(2 x)+C∫cos(x)2dx=x2+14sin(2x)+C