Question

cos(-theta)=_____
I need it fastly .

Answer 1

cos(-theta) = cos (+theta)
because theta is negative when it is measured clockwise with respect to x-axis.
And we know that cos function remains positive in fourth quadrant.
So, cos(-theta) becomes cos(theta).

Answer 2

Hey !

Solution:

Cos ( - Ф ) = ?

Well If we plot the graph for Cos Ф, it would be a graph starting from 1 in the y axis and continues till the maxima and again drops below the origin till the minima and again meets 1 at 180°.

So the graph would like the picture in the attachment.

So If we reflect the graph of Cos Ф on the 2nd quadrant keeping the y axis as the reflecting surface, we get the same image laterally inverted.

Hence we can say that the graph has same values even if the angle is negative with same magnitude.

So it implies that Cos ( - Ф ) = Even function.

Hence Cos ( - Ф ) = Cos Ф

Hope my answer helped !

Refer Attachment fro graph !