6. Find the number among natural number 1
to 288 for which the sum of the numbers
smaller than it is same as the sum of the
numbers greater than it.
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Let the number be n.
So:
1+2+3................n-1 = n+1+n+2......................288
In the first AP,
Sn=n/2*[2a+(n-1)d]
no. of terms=n-1
a=1
d=1
Sn=(n-1)/2*[2+n-1-1]
=(n-1)/2*[n]................(1)
Second AP
a=n+1
d=1
no. of terms=288-n
Sn=n/2*[2a+(n-1)d]
Sn=(288-n)/2*[2+288-n-1]
=(288-n)/2*[n+289]..............(2)
(1)=(2)
So:
(n-1)/2*[n]=(288-n)/2*[n+289]
Multiplying both sides by 2.
==) n(n-1)=(288-n)(289+n)
==)n²-n=83232+288n-289n-n²
==)2n²-n=83232-n
==)2n²=83232
==)n²=83232/2
==)n²=41616
==)n=204
n cannot be negative so n is not -204
The term hence is 204
Hope it helps you.
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