Question

why cos has not negative sign

Answer 1

Signs (plus/minus) of Sine and Cosine in all Quadrants

The number zero has no sign: 0 is not positive and 0 is not negative. Zero is neutral! For example, cosπ2=cos90∘=0 ⁡ π 2 = cos ⁡ 90 ∘ = 0 , hence has no sign. If a number is not defined, then (of course) it has no sign.

Answer 2

Answer:

cos(-a)= cos a

because angle are positive when its moving anticlockwise.and negative when moving clockwise.

if angle in negative, its definitely in 4th quadrant if a

$\alpha < 90$

so cos is positive in 1st and 4th quadrant .so cos(-a)=cosa