Question

which expression is equivalent to cos120?


cos60
cos240
coss300
cos420

Answer 1

The correct answer for the given question is option b) cos 240 degrees.

This means that the expression cos 120 degrees can also be written as cos 240 degrees.

This because cos 240 degrees = -1/2 = cos 120 degrees.

This happens because the second and third quadrants carry the same signs.

Answer 2

Answer:

Option B is Correct.

Step-by-step explanation:

Given: cos 120

To find Equivalent Expression for cos 120

Equivalent Expression for cos 120 is the expression whose value is same as cos 120.

Value of cos 120 = cos ( 180 - 60 )

         = - cos ( 60 )  (∵value of cos in 2nd quadrant is negative)

         =  $\frac{-1}{2}$

Option A:

Value of cos 60 =  $\frac{1}{2}$

⇒ Value of cos 60 ≠ Value of cos 120

Thus, This is not correct option.

Option B:

value of cos 240 = cos ( 180 + 60 )

            = - cos ( 60 )   (∵value of cos in 2nd quadrant is negative)

            =  $\frac{-1}{2}$

⇒ Value of cos 60 = Value of cos 120

Thus, This is correct option.

Option C:

value of cos 300 = cos ( 360 - 60 ) = cos ( 2×180 - 60 )

                            = cos ( 60 )  =  $\frac{1}{2}$

⇒ Value of cos 300 ≠ Value of cos 120

Thus, This is not correct option.

 

Option D:

value of cos 420 = cos ( 360 + 60 ) = cos ( 2×180 + 60 )

                            = cos ( 60 )  =  $\frac{1}{2}$

⇒ Value of cos 420 ≠ Value of cos 120

Thus, This is not correct option.

Therefore, Option B is Correct.