Question

find the sum of last ten term of AP is 8,10,12............126

Answer 1

given: a = 8
d= 10-8= 2
tn= 126
tn = a+ ( n-1 ) d
126 = 8+(n-1)2
126-8=2n- 2
118=2n-2
118+2=2n
120=2n
120/2=n
60=n
n= 60
there are 60 terms in A.P.
last ten term means (60-9) = 51 th term.
sn = n/2 {2a+(n-1)d}
= 51/2{2×8+(51-1)2}
=51/2{16+(50)2}
=51/2{16+100}
=51/2{116}
=51×58
= 2958
the sum of last ten terms is 2958