cosa + cos2a + cos6a + cos7a
(cosa+cos7a)+(cos2a+cos6a)
2cos4acos3a+2cos4acos2a
2cos4a*(cos3a+cos2a)
2cos4a*2cos(5a/2)cos(a/2)
4cos4a*cos(5a/2)*cos(a/2)
cosa + cos2a + cos6a + cos7a
(cosa+cos7a)+(cos2a+cos6a)
2cos4acos3a+2cos4acos2a
2cos4a*(cos3a+cos2a)
2cos4a*2cos(5a/2)cos(a/2)
4cos4a*cos(5a/2)*cos(a/2)