Step-by-step explanation:
(5.2+3.1)+7.4=5.2+(3.1+7.4)
(8.3)+7.4=5.2+(10.5)
15.7=15.7
hence,L.H.S =R.H.S PROVED
now let's recall that a full circle has 360°, so then 1140 = 360 + 360 + 360 + 60.
namely, 1140 is 3 full-go-around the circle and then it lands on the I Quadrant at 60°. Thus cos(1140°) = cos(60°).
what's that cosine of 60°? Check the picture below.
y/5 = x
multiply by 5 on each sides
y = 5x
There you go! I really hope this helped, if there's anything just let me know! :)