Question

In an AP the sum of first 6 terms is 5 times the sum of next 6 terms if its first term is 20 find the common difference of the AP.

Answer 1

5S6= S12
a= 20
d=?
S6= 3(2*20+(6-1)d)
=3(69+5d)
=207+15d
S12= 6(2*20+(12-1)d)
=6(40+11d)
=240+66d
5(207+15d)=240+66d
1035+75d= 240+66d
1035-240=66d-75d
795=-9d
d= -88
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