Sequence and Series question (1 Viewer)

[buckskinbrumby]

"The track of a gramophone record is in the shape of a spiral curve and may be considered as a number of concentric circles of inner and outer radius 5.25cm and 10.5cm respectively. The record rotates at 33 1/3 rev/min and takes 18 minutes to play. Find the length of the track (take pi=22/7)"

What the what?? It's an arithmetic series, but not sure how to go about it...help please? Thanks

 

[Nailgun]

33 1/3 revolution/min with 18 mins implies that there are 600 revolutions to be made, i.e. there are 600 circles

circumference of a circle is 2(pi)r, so

Sn = n/2(a+l)

n=600, a = 2*(22/7)*5.25, l=2*(22/7)*10.5

I ceebs finding a calculator, but I think thats right

 

[buckskinbrumby]

33 1/3 revolution/min with 18 mins implies that there are 600 revolutions to be made, i.e. there are 600 circles

circumference of a circle is 2(pi)r, so

Sn = n/2(a+l)

n=600, a = 2*(22/7)*5.25, l=2*(22/7)*10.5

I ceebs finding a calculator, but I think thats right

thanks!! =D