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Sin[(a - b)*x]/(2*(a - b)) + Sin[(a + b)*x]/(2*(a + b))

Integrate[Cos[a*x]*Cos[b*x], x] ==
Sin[(a - b)*x]/(2*(a - b)) + Sin[(a + b)*x]/ (2*(a + b)) 

Integrate[Cos[a x] Cos[b x], x] == 

Sin[(a - b) x]   Sin[(a + b) x]
-------------- + --------------
  2 (a - b)        2 (a + b)
Time to compute: 0.13 second
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