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GCSE Maths Question.

Hey everyone,
Got this question which shouldn't be troubling me but I can't get my head around it:

"Here is the net of a cone



The net is a sector of a circle, centre 'O', and radius 15 cm.
Reflex angle AOB = 216 degrees.
The net makes a cone of slant height 15 cm.

Work out the vertical height of the cone."

I hope that's clear enough for everyone, sorry about the quality (tried to do it quickly in paint).
I've got an idea of the formulas to use, but I'm going to keep my mouth shut to see how close I was to the proper answer.

Any help will be much appreciated. Thanks a lot.
Reply 1
Oh this was on a calculator paper as well by the way...
You find the circumference of the whole circle.
pi * diameter = 30π

You then find the circumference of the circle that will make up the base of the cone which is: 30π * (216/360)

You then work out the radius of this circle
which is: (30π * (216/360))/2π

You then use Pythagoras' rule to find the vertical height.
+ =
which is c²/a² =
so ((30π * (216/360))/2&#960:wink:² / 15² = vertical height

I dont have a calculator but that should give you the correct answer
Reply 3
nklm
It will actually be c2-a2= b2 not c2/a2=b2 but everything else is correct!