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RSY@ @pþÝn¾aì@pbÉibnn¾a
S8 = 8 (2a1 + 7 × 6)
2
440 = 4(2a1 + 42)
2a1 = 68
a1 = 34
.a3 = 34 + 2 × 6 = 46
−16 :( ) ()
.6
. S 37 = 37 2 × (−16) + 36 × 6 = 37 × 92 = 3404
2
:( ) ()
a + (a + d) + (a + 2d) = −300
( ) 3a + 3d = −300
a + (a + d) + + (a + 8d) = 300
9a + 36d = 300
( ) 3a + 12d = 100
. 6a + 15d .a + (a + d) + + (a + 5d)
. 9d = 400 () ()
6a + 15d = 6a + 6d + 9d ()
= 2(3a + 3d) + 9d
= 2 ×(−300) + 400 = −200
:( )
. ar5 = 2 ⇒ r2 = 5
ar 7 5 2