tolong jwb yg dilingkari ya

cos (a+b) cos(a-b) + 1
= 1/2[cos((a+b)+(a-b)) + cos((a+b)-(a-b))] + 1
= 1/2[cos 2a + cos 2b] + 1
= 1/2[(2cos^2 a - 1) + (2cos^2 b - 1)] + 1
= 1/2[2cos^2 a + 2cos^2 b - 2] + 1
= cos^2 a + cos^2 b - 1 + 1
= cos^2 a + cos^2 b
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