Help? Please?

[xXRUSHXx]

Okay, so I was working through a past maths paper and came across a question I was unsure about. You needed to prove that an angle equalled 120. So I checked the marking scheme and it said 360 - 2*90 - 60. Then on the side it said "The 2 90's and the 60 must be clearly justified". I was wondering how I would do this?

It was this paper here:

http://xtremepapers.com/CIE/Cambridge%2 ... _qp_41.pdf

And this is the marking scheme:

http://xtremepapers.com/CIE/Cambridge%2 ... _ms_41.pdf

It is question 7c and any help would be greatly appreciated.

Thank you in advance.

 

[waleedsmz]

Ok so you have two radius perpendicular on straight edges of the plastic sheet.

1- Since the plastic sheet sides cover equal lengths of cylinders then all of the sides are equal.

2- If you extend the straight edges of the plastic sheet to form a triangle, it will be an equilateral one and ever angle would be equal 60 ( proved in 1 )

3- Now you have a little quad at the left corner. With an angle of 60 and two angles of 90, the fourth angle will be equal to 120.

Hope that helped. : )