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arith progress first 15 terms

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by amising6 » Wed Jun 16, 2010 9:16 am
now Arithmetic progression sum formula
s=(n/2)(2*a+(n-1)d)
a= first term
d= common difference

term of an AP t=a+(n-1)d
t(4)=a+3d
t(12)=a+11d
given t(4)+t(12)=20
a+3d+a+11d=20
2a+14d=20 ------( equation 1)

we know
s=(n/2)(2*a+(n-1)d)
s=(15/2)(2a+(15-1)d) (taking n as 15 as we need to find sun of 15 terms)
s=(15/2)(2a+14d)
s=(15/2)*20 (replacing value of 2a+14d from equation 1)
s=150
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