the sum of first 11 terms of an AP is 19 and sum of first 19 terms is 11.find the sum of first 30 terms
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Step-by-step explanation:
a11= 19 that. is a+10d
A19 = 11 that is a+18d 2
a+10d=19
a=19-10d. 1
sunstituting The value of 1 in 2
therefore
a+18d=11
19-10d+18d=11
19+8d=11
8d =11-19
8d= -8
d=-8/8
d=-1
a=19-10d
= 19- 10(-1)
= 19+10
=29
s30 = n/2 ( 2a + (n-1)d)
= 30/2 (2 (29)+ 29(-1))
= 15 ( 58-29)
= 15 ( 29)
= 435
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