Page 254 - aljaber

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a5 = a1 + 4d = 15
a25 = a1 + 24d = 105

20d = 90
d=9
2

a1 = 15 − 4×9 = 15 − 18 = −3
2

.a21 = a1 + 20d = −3 + 20 × 9 = 87
2

:( ) ( )
)
. 272 = a5 = a1r 4 = 17r 4 a1 = 17

r 4 = 272 = 16
17

r = ±2

r =2

( ) ( )S5
= a1 r5 −1 = 17 25 − 1 = 17 × 31 = 527
r −1

r = −2

( ) ( ).S5
= a1 r5 −1 17 (−2)5 − 1 = 17 × (−33) = 187
r −1 = −3

−2 − 1

Sn = 60 d = −4 a1 = 20 :( ) (

.