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a. sin 42-sin 18
=2 cos 1/2(42+18) sin 1/2(42-18)
=2 cos 30 sin 12
= 2.1/2(akar 3) sin 12
= sin 12 (akar 3)
b. cos 80 + cos 40
= 2 cos 1/2 (80+40) cos 1/2 (80-40)
=2 cos 60 cos 20
=2.1/2 cos 20
=cos 20
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