pls help i dont understnd trig

sin(1755^o) = -0.707106781187       via calculator

1755/360 = 4 and a bit

so it is 4 revolutions plus a partial revolution

4*360=1440

1755-1440=315

so

Sin 1755 = sin 315

315 degrees is in the 4th quadrant where sin is always negative.

360-315=45 degrees

sin 315 = -sin 45 degrees 

Draw a right angled, isosceles triangle with the legs being 1 unit in length.  

the angles are 90, 45 and 45 degrees

Use pythagoras's Theorum to find the hypotenuse.

From that you can work out what sin45⁰  is.