In an AP if the sum of 4th and 8th term is 70 and its 15th term is 80 then find the sum of the first 25th terms
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a15=a+14d
a15-14d=a. equation1
a4+a8=(a+3d)+(a+7d)
70=80-14d+80-14d+10d From eq 1
70=160-28d+10d
70-160= -18d
-90/-18=d
5=d
Put in eq 1
80-14(5)=a
80-70=a
10=a
s25=n/2{2a+(n-1)d}
s25=25/2{2(10)+(25-1)5}
s25=25/2{20+(24*5)}
s25=25/2(20+120)
s25=25/2*140
s25=25*70
s25=1750
a15-14d=a. equation1
a4+a8=(a+3d)+(a+7d)
70=80-14d+80-14d+10d From eq 1
70=160-28d+10d
70-160= -18d
-90/-18=d
5=d
Put in eq 1
80-14(5)=a
80-70=a
10=a
s25=n/2{2a+(n-1)d}
s25=25/2{2(10)+(25-1)5}
s25=25/2{20+(24*5)}
s25=25/2(20+120)
s25=25/2*140
s25=25*70
s25=1750
Adityadang:
Easiest way to do this answer
a15=80
I don't know easiest way to give the answer of this question
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