Let be an AP such that . If the sum of this AP is 189 , then is equal to...

(d): Let aligned& t_n=1d(1a_n-1a_n+1) \\& _n=1^20 t_n=1d(1a_1-1a_2+1a_2-1a_3 +1a_20-1a_21) \\& =1d(1a_1-1a_21)=(a_21-a_1)d a_1 a_21=20 dd a_21 a_1=20a_1 a_21=49alignedaligned& a_1 a_21=45 \#(i) \\& 189=212(a_1+a_21) a_1+a_21=18 \\& Using (i)) \\& a_1+45a_1=18 a_1^2-18 a_1+45=0 \\& (a_1-15)(a_1-3)=0 ...

Let a_{1},a_{2},,a_{21} be an AP such that \sum_{n = 1}^{20}\mspace{2mu}\frac{1}{a_{n}a_{n + 1}} = \frac{4}{9}. If the sum of this AP is 189 , then a_{6}a_{16} is equal to: Sequence & Series (Mathematics)

Let

a_{1},a_{2},\ldots,a_{21}

be an AP such that\n

\sum_{n = 1}^{20}\mspace{2mu}\frac{1}{a_{n}a_{n + 1}} = \frac{4}{9}

.\n If the sum of this AP is 189 , then

a_{6}a_{16}

is equal to

Let A_{1},G_{1} and H_{1} denote the arithmetic, geometric and harmonic means . respectively of two distinct positive numbers. For n ? 2 let A_{n - 1} and H_{n - 1} have . arithmetic, geometric and harmonic means as A_{n},G_{n},H_{n} respectively. Which of the following statements is correct?

If a_{1},a_{2},a_{3}, are in A.P. such that a_{1} + a_{7} + a_{16} = 40, then the sum of the first 15 terms of this A.P. is

For any odd integer n ? 1,n^{3} - (n - 1)^{3} + + ( - 1)^{n - 1}1^{3} =

Let a,b be two non-zero real numbers. If p and r are the roots of the equation x^{2} - 8ax + 2a = 0 and q and s are the roots of the equation \end{enumerate} x^{2} + 12bx + 6b = 0, such that \frac{1}{p},\frac{1}{q},\frac{1}{r},\frac{1}{s} are in A.P., then a^{- 1} - b^{- 1} is equal to

If a_{n} = \frac{3}{4} - ( \frac{3}{4} )^{2} + ( \frac{3}{4} )^{3} + ( - 1)^{n - 1}( \frac{3}{4} )^{n} and b_{n} = 1 - a_{n}, then find the least natural number n_{0} such that b_{n} > a_{n}\forall n ? n_{0}

If a_1, a_2, a_n are in A.P. where a_i>0 for all i show that &\frac{1}{\sqrt{a_1}+\sqrt{a_2}}+\frac{1}{\sqrt{a_2}+\sqrt{a_3}}+\cdots+\frac{1}{\sqrt{a_{n-1}}+\sqrt{a_n}}&=\frac{n-1}{\sqrt{a_1}+\sqrt{a_n}}

Different A.P. are constructed with the first term 100 , the last term 199, and integral common differences. The sum of the common differences of all such A.P. having at least 3 terms and at most 33 terms is

The sum of the first n terms of the series (1^{2} + 2.2^{2} + 3^{2} + 2.4^{2} + 5^{2} + 2 . 6^{2} + ..) is (\frac{n(n + 1)^{2}}{2}, when n is even. when n is odd, the sum is

Number of 4 -digit numbers that are less than or equal to 2800 and either divisible by 3 or by 11 , is equal to

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