Page 228 - aljaber

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a4 + a5 + + a13 + a14 = 77 a
⇒ 11a + 88d = 77
⇒ a + 8d = 7
⇒ a = 7 − 8d

3a + 18d = 17

3(7 − 8d) + 18d = 17
21 − 24d + 18d = 17

−6d = −4
d=2
3

.a = 7 − 8× 2 = 5

33
ak = 13 ⇔ a + (k − 1)d = 13

⇔ 5 + (k − 1) × 2 = 13
33

⇔ k − 1 = 17

.k = 18

12 3 ()
.

. d
. 3, 3 + d, 3 + 2d, 3 + 3d, 3 + 4d,12