the sum of first five terms of an ap and sum of the first seven terms of ap is 167 if the sum of the first ten teems of this ap is 235 find the sum of its first twenty terms
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the sum of first five terms of an ap and sum of the first seven terms of ap is 167 if the sum of the first ten teems of this ap is 235 find the sum of its first twenty terms.
S5+S7=167
==>5/2[2a+(5-1)d]+7/2[2a+(7-1)d]
==>12a+31d=167 ----> A
S10=235
10/2[2a+(10-1)d]=235
==>10a+45d=235
==>2a+9d=47 ---->B
on solving eq A, & B
a=1,
d=5
S20=20/2[2(1)+(20-1)*5]
S20=970
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