Respuesta :

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Answer:

sin(a + b) = sinacosb + cosasinb;

tana = 20/21, then cosa = - 1/√1 + (20/21)2 = - 21/29, sina = tanacosa = 20/21·(- 21/29) = - 20/29;

sinb = √1 - (- 4/5)2 = 3/5

sin(a + b) = - 20/29·(-4/5) + (- 21/29)·3/5 = 80/145 - 63/145 = 17/145

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