What is the identity of cos 2 θ

1 answer:

7 0

Using the cosine double-angle identity. The cosine double angle formula tells us that cos(2θ) is always equal to cos²θ-sin²θ.

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$S_1$ happens to be the first term in the series, which is why that box is marked correct:

$S_1=\displaystyle\sum_{n=1}^1\frac3{(-2)^n}=\frac3{(-2)^1}=-1.5$

But the next partial sum is not correct:

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