Product Sum and Sum Product Formulas in Trigonometry

1. sin x cos y = (1/2) [ sin(x + y) + sin(x - y) ]
   
   
2. cos x sin y = (1/2) [ sin(x + y) - sin(x - y) ]
   
   
3. cos x cos y = (1/2) [ cos(x + y) + cos(x - y) ]
   
   
4. sin x sin y = (1/2) [ cos(x - y) - cos(x + y) ]

Sum to Product Formulas

1. sin x + sin y = 2*sin[ (x + y) / 2 ]*cos[ (x - y) / 2 ]
   
   
2. sin x - sin y = 2*cos[ (x + y) / 2 ]*sin[ (x - y) / 2 ]
   
   
3. cos x + cos y = 2*cos[ (x + y) / 2 ]*cos[ (x - y) / 2 ]
   
   
4. cos x - cos y = 2*sin[ (x + y) / 2 ]*sin[ (x - y) / 2 ]

• Trigonometric Formulas and Their Applications
  
  
• Trigonometric Identities and Their Applications