急!F6 Arithmetic Sequences 01q8

9.
The time of reading (in minutes) on the nth day forms an arithmetic sequence,with
the first term, a = 30
the common difference, d = 10
the number of terms, n = 7

S7
= 7 x (2a + 6d) / 2
= 7 x (2 x 30 + 6 x 10) / 2
= 420

Total time that Fanny spends on reading in a week
= 420 minutes
= 420/60 hours
= 7 hours


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13.
Let (r - d) cm, r cm and (r + d) cm be the radius of C1, C2and C3 respectively.

The sum of the three diameters :
2 x [(r - d) + r + (r + d)] = 18
3r = 9
r = 3

The area of C3 :
π(r + d)² = 16π
(3 + d)² = 16
3 + d = 4 or 3 + d = -4 (rejected)
d = 1

The diameter of C3
= 2 x (3 + 1) cm
= 8 cm

(b)
The diameter of C1
= 2 x (3 - 1) cm
= 4 cm

The diameter of C2
= 2 x 3 cm
= 6 cm