70 4 5
a , ar , ar2
a + ar + ar2= 70
a(1 +r +r2 ) = 70
.......(1) a =
4a , 5ar , 4ar2
5ar- 4a = 4ar2 -5ar
10ar -4a -4ar2 = 0
a( 10r -4 - 4r2 ) = 0 .......(2)
1 2
(10 r -4 -4r2 ) = 0
2r2 -5r + 2 = 0
( 2r -1 ) ( r - 2 ) =0
either r =
a= 40
40 , 20 , 10
or r = 2 , a = 10
10 , 20 , 40
; 12-24-2012 10:19 PM
: 1 (0 1 )
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