sederhanakanlah 2 cos (x + 20) cos (x-10)

TriGonoMetRi

2 cos a cos b = cos (a + b) + cos (a - b)

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2 cos (x + 20) cos (x - 10)
= cos ((x + 20) + (x - 10)) + cos ((x + 20) - (x - 10))
= cos (2x + 10) + cos 30