sin(nπ+2/3π)*cos(nπ+4/3π)
=(sinnπcos2/3π+cosnπsin2/3π)(cosnπcos4/3π-sinnπsin4/3π)
=(0+√3/2cosnπ)(-1/2cosnπ-0)
=-√3/4(cosnπ)²
=-√3/4