1.

If `sum_(r=1)^(n) r(sqrt(10))/(3)sum_(r=1)^(n)r^(2),sum_(r=1)^(n) r^(3)` are in G.P., then the value of n, isA. 2B. 3C. 4D. non-existent

Answer» Correct Answer - C
We have,
`(10)/(9)(underset(r=1)overset(n)sumr^(2))^(2)=(underset(r=1)overset(n)sumr)(underset(r=1)overset(n)sumr^(3))`
`rArr" "(10)/(9){(n(n+1)(2n+1))/(6)}^(2)=(n(n+1))/(2)xx{(n(n+1))/(2)}^(2)`
`rArr" "(10)/(9)((2n+1)/(3))^(2)=(n(n+1))/(2)`
Clearly, n=4 satisfies this equation.


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