120, 315, 300, 345, 720
120, 315, 300, 345 = 15( 8, 21, 20, 23)
Let Un = an³ + bn² + cn + d
U-2 = -8a + 4b - 2c + d = 8 .............. (1) Note the translation by 3 places back. This will be rectified by retranslating 2 forward at the solution by substituting (n - 3) for n
U-1 = -a + b - c + d = 21 ................. (2)
U0 = d = 20 .................... ................. (3)
U1 = a + b + c + d = 23 .................. (4)
Substitute (3) into (1), (2) and (4) and simplify
8a - 4b + 2c = 12
ie 4a - 2b + c = 6 ........... ................. (5)
a - b + c = -1 ................. .................. (6)
a + b + c = 3 ................. .................. (7)
(5) + 2*(7) 6a + 3c = 12
ie ............... 2a + c = 4 ................... (8)
(6) + (7) ... 2a + 2c = 2
ie ................ a + c = 1..................... (9)
(8) - (9) a = 3
So c = -2 and b = 2
So a = 3, b = 2, c = -2, d = 20
So Un+3 = 3n³ + 2n² - 2n + 20
So Un = 3(n - 3)³ + 2(n - 3)² - 2(n - 3) + 20
= 3(n³ - 9n² + 27n - 27) + 2(n² - 6n + 9) - 2n + 6 + 20
= 3n³ - 27n² + 81n - 81 + 2n² - 12n + 18 - 2n + 26
= 3n³ - 25n² + 67n - 37
So Tn = 15*Un
ie Tn = 45n³ - 375n² + 1005n - 555
Check on Excel
n ................ 1 ............ 2 ........... 3 ........... 4 ........... 5
Un .............. 8 .......... 21 ......... 20 ......... 23 ......... 48
Tn .......... 120 ......... 315 ....... 300 ....... 345 ....... 720
Addendum
if all that is important is the term then you can readuily and easily get it by method of differences. First set up the generations of differences
120 ...... 315 ...... 300 ...... 345
...... 195 ...... -15 ........ 45
............ -210 ....... 60
.................... 270
Then climb the right hand side of the difference set starting with the sme difference if you want to maintain the same order ... if you want to increase the order of Tn by 1 then start with a different number (any one you like in fact!!) So any answer is correct But the real question is to find Tn not the next number in the sequence.
120 ...... 315 ...... 300 ...... 345 ....... 720
...... 195 ...... -15 ........ 45 ....... 375
............ -210 ....... 60 ...... 330
.................... 270 ..... 270 ← start here and work back up by addition of number above left so 330 = 270 + 60, etc
2006-11-02 08:24:18
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answer #1
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answered by Wal C 6
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2016-12-05 11:32:22
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answer #2
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answered by puiatti 4
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728
2006-11-02 06:29:27
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answer #3
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answered by ramesh the great 1
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150. As in 120,315,300,345,150,375.
The sequence is 15^1,^13, ^(-1), ^3, .....
2006-11-02 07:07:55
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answer #4
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answered by Anonymous
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550?
2006-11-02 06:27:24
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answer #5
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answered by Anonymous
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there is no logical # in the sequence. You can insert any number you want and it will make as much sense as any other #.
2006-11-02 06:26:10
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answer #6
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answered by USMCstingray 7
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120,315,300,345,390
2006-11-02 06:30:18
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answer #7
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answered by astridmarie_y2k 2
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