The sum of first 5 terms of an ap is 45 and that of its first 15 is 435. Find the sum of first n terms of this ap
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S(5)= 45
5/2 ( 2a +(5-1)d )=45
2a+4d=18
a+2d=9--------1st eqn
S(15)=435
15/2 ( 2a+(15-1)d ) = 435
2a+14d=58
a+7d=29-----------2nd eqn
Solving 1st and 2nd equation
d=4
a=1
So ap is:
1 , 1(4), 1(4)(4)....
Sum to n terms:
N/2 ( 2 + (n-1)4 )
n/2 ( 4n-2 )
5/2 ( 2a +(5-1)d )=45
2a+4d=18
a+2d=9--------1st eqn
S(15)=435
15/2 ( 2a+(15-1)d ) = 435
2a+14d=58
a+7d=29-----------2nd eqn
Solving 1st and 2nd equation
d=4
a=1
So ap is:
1 , 1(4), 1(4)(4)....
Sum to n terms:
N/2 ( 2 + (n-1)4 )
n/2 ( 4n-2 )
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