Page 256 - aljaber

RTS@ @pþÝn¾aì@pbÉibnn¾a

a1 = S1 = 2 × 12 + 6 × 1 = 8

a2 = S2 − a1 = 2 × 22 + 6 × 2 − 8 = 12

.d = a2 − a1 = 12 − 8 = 4

a3 = a1 + 2d = 8 + 2 × 4 = 16

a15 = a1 + 14d = 8 + 14 × 4 = 64
a3, at , a15

at2 = a3 ×a15 = 16 × 64

.at = 4 × 8 = 32

32 = at = 8 + (t − 1)× 4
4t = 28

t = 28 = 7
4

:( ) ( )
)
. 64, 61, 58, … −3 64

. a18

.a18 = a1 + 17d = 64 + 17 × (−3) = 13

.a1r 6 = 160 a1r 2 = 10 :( ) (

a1r 6 = 160
a1r 2 10

r 4 = 16

r = ±2

.a2 = a1r = 5×2 = 5 a1 = 5 r =2
2 2