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:( ) ()
()
71 − 7n < −200 :( )
2 ()
()
7n > 471

n > 471 ≈ 67.3
7

. 68 n
.a − d, a, a + d

a − d + a + a + d = 21
3a = 21
a=7

7(7 + d) = 2 ×(7 − d)× 7

7 + d = 14 − 2d

d=7
3

. 7 + 7 = 28

33

.2 3 :( )

Sn = n  2 × 3 + (n − 1) × 2  = n (4 + 2n)
2 2
= 2n + n2 = n(n + 2)

.a7 = 5832 a1 = 8 :( )