Page 234 - aljaber

RRQ@ @pþÝn¾aì@pbÉibnn¾a

Sn = an + (an − d) + (an − 2d) + Sn
+ (a1 + 2d) + (a1 + d) + a1

2Sn = (a1 + an ) + (a1 + an ) + + (a1 + an )

.n

2Sn = n(a1 + an )

Sn = n (a1 + an )
2

an = a1 + (n − 1)d

Sn = n (2a1 + (n − 1)d)
2

. 40 5 + 8 + 11 + 14 + … ()

.n = 40 d = 3 a1 = 5

. S 40 = 40 (2 × 5 + 39 × 3) = 20(10 + 117) = 2540
2

. 50 + 49 1 + 49 + 48 1 + + (−20) ( )

22

.an = −20 d = −1 a1 = 50
2