find the sum of n terms of two AP are in the ratio 5n + 4 : 9n -6 and find the ratio of their 18th term
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If you observe the Sn of AP its actually of the form An²+Bn right? Can't see it? Okay wait...
Sn = n/2(2a + (n-1)d)
= an + n²d/2 - nd/2
= (d/2)n² + (a - (d/2))n
by comparison A = d/2 and B = (a - (d/2))
when n=1 i.e. the first term Sn = a and you can see that by putting n as 1... d/2 gets cancelled.
d is 2*(coefficient of n²) right?
I've done the same things in the solution. If still in doubt, let me know...
Godspeed!!
Sn = n/2(2a + (n-1)d)
= an + n²d/2 - nd/2
= (d/2)n² + (a - (d/2))n
by comparison A = d/2 and B = (a - (d/2))
when n=1 i.e. the first term Sn = a and you can see that by putting n as 1... d/2 gets cancelled.
d is 2*(coefficient of n²) right?
I've done the same things in the solution. If still in doubt, let me know...
Godspeed!!
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