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arggh series (1 Viewer)
- Thread starter joannee--
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watatank
=)
the sum of the first 5 terms of an arithmetic series is 110 and the sum of the first 10 terms is 320. Find the first term and the common difference.
Sn = (n/2) * [2a + (n-1)d ]
S(5) = (5/2) * (2a + 4d) = 110
10a + 20d = 220 .................. (equation 1)
S(10) = (10/2) * (2a + 9d) = 320
20a + 90d = 640 .................. (equation 2)
solve equation 1 and 2 simultaneously
EDIT: because im bored il finish it off
10a + 20d = 220 .................. (equation 1)
2 * (equation 1) ----> 20a + 40d = 440 .................. (equation 3)
20a + 90d = 640 .................. (equation 2)
equation 2-equation 3: 50d = 200 ... d = 4
sub back into equation 1
10a + 80 = 220
10a = 140.... a = 14
Sn = (n/2) * [2a + (n-1)d ]
S(5) = (5/2) * (2a + 4d) = 110
10a + 20d = 220 .................. (equation 1)
S(10) = (10/2) * (2a + 9d) = 320
20a + 90d = 640 .................. (equation 2)
solve equation 1 and 2 simultaneously
EDIT: because im bored il finish it off
10a + 20d = 220 .................. (equation 1)
2 * (equation 1) ----> 20a + 40d = 440 .................. (equation 3)
20a + 90d = 640 .................. (equation 2)
equation 2-equation 3: 50d = 200 ... d = 4
sub back into equation 1
10a + 80 = 220
10a = 140.... a = 14
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