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AP nth term

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AP nth term

by maihuna » Sun May 10, 2009 7:31 am
In an A.P., the first term is 2 and the sum of the first five terms is one-fourth of the next five terms. Find the 20th term.

-112
-57
-55
0
55
112
Charged up again to beat the beast :)
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by scoobydooby » Sun May 10, 2009 10:14 am
1st 5 terms: 2, 2+d, 2+2d,......,2+4d
=>sum of 1st 5 terms=10+10d

sum of next 5 terms: (2+5d)+(2+6d)+...+(2+9d)=10+35d

given that 10+10d=(10+35d)/4
=>d=-6

20th term: 2+19d=2+19*-6=-112

hence, A
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by dtweah » Mon May 11, 2009 5:18 am
S5=1/4(S10-S5)
5S5=S10
5 5/2(4+4d)=10/2(4+9d)
10+10d=4+9d
d=-6
t20=2+19x-6
-112.
Choose A
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