Question

Which of the following expressions is equal to cos125°?
-cos55°
cos55°
cos(-55)°

Answer 1

Answer: First option is correct.

Explanation:

Since we have given that

`\cos 125^\circ`

We need to find the value of above expression:

`\cos(\pi-\theta)=\cos(180^\circ-125^\circ)=-\cos 55^\circ`

Since π-Ф belongs to Second quadrant.

And we know that cosine is negative in this quadrant.

So, it would be -cos 55°.

Hence, First option is correct.

Answer 2

-cos(55°)

Explanation:

The reference angle for second-quadrant angle 125° is (180-125)° = 55°. The cosine is negative in the second quadrant, so the equivalent expression is ...

cos(125°) = -cos(55°)

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Your calculator (in degrees mode) can help you sort this out.