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Evaluate cos 120° without using a calculator.A.NICOOB. - 1/2O C. 1/2OD. -³

Evaluate Cos 120 Without Using A CalculatorANICOOB 12O C 12OD class=

Sagot :

Explanation

We are required to evaluate cos 120° without using a calculator.

This is achieved thus:

We know that 120° lies in the second quadrant.

We also know that cosine is negative in the second quadrant.

Therefore, we have:

[tex]\begin{gathered} \cos120\degree=\cos(180\degree-60\degree) \\ \Rightarrow-\cos60\degree \\ \text{ We know that }\cos60\degree=\frac{1}{2} \\ \\ \therefore-\cos60\degree=-\frac{1}{2} \end{gathered}[/tex]

Hence, the answer is:

[tex]-\frac{1}{2}[/tex]

Option B is correct.

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