Finding Radius Of Cone (1 Viewer)

[mudcake42]

A question of a year 10 book (so should be easy) that I can figure out as I'm revising for year 11!

A cone is made from a sector of a circle with radius of 8cm which subtends an angle of 216degrees at the centre. Find:

a) length of the arc of the circle which forms the circumference of the base ... which is 30.16

b) the radius of the base of the cone ... how do you figure that out? (answer is 4.8cm)

Cheers guys!

 

[z600]

I dont see how the radius is 4.8, it states that the radius was 8. very confusing problem though....

 

[SoulSearcher]

The radius of the circle from which the sector was cut out from has radius 8cm.

 

[z600]

ooo, got it now, didnt know what subtend means

 

[mudcake42]

nah soulsearcher... 8cm is the slope. but cheers guys for helping (specially k02033)!!!

 

[z600]

Which textbook is this?

 

[SoulSearcher]

nah soulsearcher... 8cm is the slope. but cheers guys for helping (specially k02033)!!!

Which is the radius of the circle from which the sector was cut out from

 

[Danitel]

ingore my horrible drawing....=.=.... psA- picture A.

ok,mudcake42, in PA, the question metions that the cone is made from a sector *****.....
which means the radius ( AB) of sector in PA should be equal to the length of AD (as you roll the sector up to form an cone....)

the answer you got in a) is correct....by doing the calculation 8*2*π*(216/360)= 30.16....

but you only miss out one step in b) that is...

when a cone is formed by a sector, the perimeter of the sector in PA is the perimeter of the base of the cone in PB (which is circle O).....

when you know the perimeter of the a cirlce ,what should you do to get the radius of this circle huh?

^^...hope that can help you little boy....:wave: ​