Page 35 - Ebook Trigonometri_Krisna Hasugian_MESP 2019
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Product to Sum Formulas
1
cos cos = [cos( + ) + cos( − )]
2
1
sin sin = [cos( − ) − cos( + )]
2
1
sin cos = [sin( + ) + sin( − )]
2
1
cos sin = [sin( + ) − sin( − )]
2
Example 36 Use the product-to-sum formula to change sin 75° sin 15° to a sum
1 1
sin 75° sin 15° = [cos(75° − 15°) − cos(75° + 15°)] = [cos 60° − cos 90°]
2 2
1 1 1
= [ − 0] =
2 2 4
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