Sum of first five terms an ap is 55and sum first ten terms ap is 235 find sum first20 terms
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a5=a+4d=55 ......(1)
a10=a+9d=235 ......(2)
(2)-(1)
a+9d=235
- a+4d=55
_____________
5d=180
d=36
therefore on equating the value of d on (1) or (2)
a=-89
Sn=n/2[2a+(n-1)d]
S20=10[-178+324]
=10×146
=1460
a10=a+9d=235 ......(2)
(2)-(1)
a+9d=235
- a+4d=55
_____________
5d=180
d=36
therefore on equating the value of d on (1) or (2)
a=-89
Sn=n/2[2a+(n-1)d]
S20=10[-178+324]
=10×146
=1460
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