in an AP the 9th term is 4 times the 5th term the 7th term is 17 find the sum of the first 20 terms
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let 'a' and 'd' be the first and last term of the A.P respectively..
Given: a9=4*a5 and a7=17
the nth term of an AP is given by the formula an=a+(n-1)d
(a+8d)=4(a+4d) and a+6d=17
3a+8d=0 and 3a+18d=51
a=-17 and d=51/8
now sum to n terms of an AP is given by
Sn=n/2{2a+(n-1)d}
required
S20=20/2{2(-17)+(20-1)51/8}=871.25(answer)
Given: a9=4*a5 and a7=17
the nth term of an AP is given by the formula an=a+(n-1)d
(a+8d)=4(a+4d) and a+6d=17
3a+8d=0 and 3a+18d=51
a=-17 and d=51/8
now sum to n terms of an AP is given by
Sn=n/2{2a+(n-1)d}
required
S20=20/2{2(-17)+(20-1)51/8}=871.25(answer)
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