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2 sinα.cosβ = sin (α+β) + sin (α-β) >>> saya sederhanaka yg kanan ya...
= sinacosb+cosasinb + ( sinacosb - cosasinb)
= sinacosb + cosasinb + sinacosb - cosasinb ..............yg ditebalin habis
= 2 sinacosb ................. terbukti