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\[Sina + Sinb = 2Sin\left( {\frac{{a + b}}{2}} \right)Cos\left( {\frac{{a – b}}{2}} \right)\]
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\[Sina – Sinb = 2Cos\left( {\frac{{a + b}}{2}} \right)Sin\left( {\frac{{a – b}}{2}} \right)\]
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\[Cosa + Cosb = 2Cos\left( {\frac{{a + b}}{2}} \right)Cos\left( {\frac{{a – b}}{2}} \right)\]
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\[Cosa – Cosb = – 2Sin\left( {\frac{{a + b}}{2}} \right)Sin\left( {\frac{{a – b}}{2}} \right)\]
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\[2{\text{ }}Sina{\text{ }}Cosb = Sin(a + b) + Sin(a – b)\]
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\[2{\text{ }}Cosa{\text{ }}Sinb = Sin(a + b) – Sin(a – b)\]
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\[2{\text{ }}Cosa{\text{ }}Cosb = Cos(a + b) + Cos(a – b)\]
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\[2{\text{ }}Sina{\text{ }}Sinb = Cos(a – b) – Cos(a + b)\]
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\[Si{n^2}a – Si{n^2}b = Sin(a + b)Sin(a – b)\]
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\[Co{s^2}a – Si{n^2}b = Cos(a + b)Cos(a – b) = Co{s^2}b – Si{n^2}a\]