The film is wrapped around the disc, and the diameter of the empty disc is 6 mm. The existing film with a thickness of .15mm is wound on the disc for 6 times. Find the total length of the film.

increase the diameter by two thicknesses per turn.

the average diameter D1 of the wafer after the first winding = (inner ring diameter d+ outer ring diameter after the first winding)/2

= (d+d+2 * .15)/2 = d+.15 = d-.15+2 * .15 * 1

the average diameter of the wafer after the second winding. 2=d+3*.15=d-.15+2*.15*2

The average diameter of the wafer after the third winding d3= (outer ring diameter after the second winding+outer ring diameter after the third winding)/2 = (D1+4 * .15+D+6 * .15)/. 6

total length = π (D1+D2+... D6)

= π (D-.15+2 * .15+D-.15 * 2+D-.15+2 * .15 * 3+... .15*6）

=π(（d-.15)6+2*.15*(1+2+3+……+6））

=π(（d-.15)6+2*.15*(6*（6+1）/2））

=π(（6-.15)6+2*.15*183)

=9π=2826mm=282.6m