Khan Academy on a Stick
Demostración: sen(a+b) = (cos a)(sen b) + (sen a)(cos b)
Demostración de la identidad trigonométrica sen(a+b) = (cos a)(sen b) + (sen a)(cos b)
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Demostración de la identidad trigonométrica sen(a+b) = (cos a)(sen b) + (sen a)(cos b)
If the above player doesn't work, you can try this direct link.