If $a, b, c$ are in $A.P.$,then $3^a, 3^b, 3^c$ shall be in

$2+3+5+6+8+9+\ldots$ to $2n$ terms $=$

Four numbers are in an arithmetic progression. The sum of the first and last terms is $8$ and the product of the two middle terms is $15$. What is the smallest number in the sequence?

Let the sum of the first three terms of an $A.P.$ be $39$ and the sum of its last four terms be $178.$ If the first term of this $A.P.$ is $10,$ then the median of the $A.P.$ is

If the sum of two extreme numbers of an $A.P.$ with four terms is $8$ and the product of the remaining two middle terms is $15$,then the greatest number of the series will be:

Find the sum of odd integers from $1$ to $2001$.