The 10th term of an A.P -27 and the 5th term is -12. What is the sum of it's 25th term
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Step-by-step explanation:
Let the first term be a and common difference be d
According to question
a+(10-1)d=-27
a+9d=-27.
a=-27-9d. -----------------(1)
a+(5-1)d=-12
a+4d=-12. ---------------(2)
Using (1) in (2)
-27-9d+4d=-12
-27-5d=-12
-5d=-12+27
-5d=15
d=-3
a=-27-9(-3)=-27+27=0
Sum of 25 terms = 25/2[2(0)+(25-1)-3]
=25/2[0+(24)-3]
=25/2[-72]
=-900
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