If the sum of $\mathrm{n}$ terms of an $\mathrm{A.P.}$ is $n P+\frac{1}{2} n(n-1) Q,$ where $\mathrm{P}$ and $\mathrm{Q}$ are constants, find the common difference.
Let $a_{1}, a_{2}, \ldots a_{n}$ be the given $\mathrm{A.P.}$ Then
${S_n} = {a_1} + {a_2} + {a_3} + \ldots + {a_{n - 1}} + {a_n} = nP + \frac{1}{2}n(n - 1)Q$
Therefore $S_{1}=a_{1}=P, S_{2}=a_{1}+a_{2}=2 P+Q$
So that $a_{2}= S _{2}- S _{1}= P + Q$
Hence, the common difference is given by $d=a_{2}-a_{1}=(P+Q)-P=Q$
Find the sum of all numbers between $200$ and $400$ which are divisible by $7.$
The four arithmetic means between $3$ and $23$ are
Let $\frac{1}{{{x_1}}},\frac{1}{{{x_2}}},\frac{1}{{{x_3}}},.....,$ $({x_i} \ne \,0\,for\,\,i\, = 1,2,....,n)$ be in $A.P.$ such that $x_1 = 4$ and $x_{21} = 20.$ If $n$ is the least positive integer for which $x_n > 50,$ then $\sum\limits_{i = 1}^n {\left( {\frac{1}{{{x_i}}}} \right)} $ is equal to.
Let $a , b , c$ be in arithmetic progression. Let the centroid of the triangle with vertices $( a , c ),(2, b)$ and $(a, b)$ be $\left(\frac{10}{3}, \frac{7}{3}\right)$. If $\alpha, \beta$ are the roots of the equation $ax ^{2}+ bx +1=0$, then the value of $\alpha^{2}+\beta^{2}-\alpha \beta$ is ....... .
Let $a_{1}, a_{2} \ldots, a_{n}$ be a given $A.P.$ whose common difference is an integer and $S _{ n }= a _{1}+ a _{2}+\ldots+ a _{ n }$ If $a_{1}=1, a_{n}=300$ and $15 \leq n \leq 50,$ then the ordered pair $\left( S _{ n -4}, a _{ n -4}\right)$ is equal to