I(n)
= [-cos(t)*(sin(t))^(n-1)] + ∫cos(t) *cos(t)*(n-1)*(sin(t))^(n-2) dt
= [-cos(t)*(sin(t))^(n-1)] + (n-1)*∫(1-(sin(t))^2) * (sin(t))^(n-2) dt
= [-cos(t)*(sin(t))^(n-1)] + (n-1)*∫(sin(t))^(n-2) dt - (n-1)*∫(sin(t))^n dt
= [-cos(t)*(sin(t))^(n-1)] + (n-1)*I(n-2) - (n-1)*I(n)
= (n-1)*I(n-2) - (n-1)*I(n)