Write out the sum formula for sin <span>sin(x + y) = sinxcos + sinycosx </span>
<span>Then expand sin(a + b) + sin(a - b) </span> <span>sinacosb + sinbcosa + sinacosb - sinbcosa </span> <span>The 2nd and 4th terms cancel and you get </span> <span>2sinacosb</span>
