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RTQ@ @pþÝn¾aì@pbÉibnn¾a
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 :( ) (
.