Cos8a cos5a - cos12a cos9a /sin8a cos5a + cos12a sin9a = tan 4a
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(cos8A.cos5A-cos12A.cos9A)/(sin8A.cos5A+cos12A.sin9A)
=> [1/2 {cos13A+cos3A}-1/2 {cos21A+cos3A}]/[1/2 {sin13A+sin3A}+1/2 {sin21A-sin3A}]
=>( cos13A-cos21A)/(sin13A+sin21A)
=> 2sin17A.sin4A/2sin17A.cos4A
=> tan4A
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