cos8A.cos5A - cos12A.cos9A / sin8A cos5A + cos12A.sin9A
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sin8Acos5A+cos12Asin9A
cos8Acos5A−cos12Acos9A
=
2
1
[(sin13A+sin3A)+(sin21Asin3A)]
2
1
[(cos13A+cos3A)−(cos21Acos3A)]
=
sin13A+sin21A
cos13A−cos21A
=
2sin(
2
13+21
)Acos(
2
21−13
)A
2sin(
2
13+21
)Asin(
2
21−13
)A
=
cos4A
sin4A
=tan4A
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