Question

find the exact value of cos(-390 degree)

Answer 1

To Find : Cos(-390 degrees)

Solution :

we Know that,

Cos (-theta) = Cos (theta)

hence, Cos(-390 Degrees) = Cos(390 Degrees)

            Cos(390 Degrees) can also be written as Cos ( 360 + 30 ) Degrees

          using Cos(A+B) Formula ,

                   Cos (A+B) = Cos A Cos B -Sin A Sin B

                   Therefore, Substituting,

Cos 360 Cos 30 - Sin 360 Sin 30

= Cos 30                                    (Since Cos 360= Cos 0 = 1 , Sin 360 = 0)

= √3/2