series question (1 Viewer)

[darshil]

here is a question from the fitzpatrick 2u maths book:
The track of a gramophone record is in the shape of a psiral curve and maybe 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 (pi=22/7)
Ive tried so hard to do this one
worked solutions are appreciated. I wish i had a gramophone.

 

[Timothy.Siu]

i'm not sure if this is right because i'm not sure if i understand the question...

ok so
18x33 1/3 = 600 (revolutions in total)
so 600 circles?

small circle is 5.25x2x22/7=a
large circle is 10.5x2x22/7=b
so then u can use sum of arithmetic series
(a+b)/2 x 600= 29700cm