Find the sum of n terms of the series 3 + 7 + 13 + 21 + 31 +....

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 Multiple Choice QuestionsShort Answer Type

131. Find the sum to n terms of the following series.

space space space 1 cross times 2 cross times 3 plus 2 cross times 3 cross times 4 plus 3 cross times 4 cross times 5 plus........
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132. Find the sum of n terms of the series 3 + 7 + 13 + 21 + 31 +.......


Let  straight S subscript straight n space equals space 3 plus 7 plus 13 plus 21 plus 31 plus......... space plus space straight T subscript straight n minus 1 end subscript space plus space straight T subscript straight n

Re-writing it as straight S subscript straight n space equals space 3 plus 7 plus 13 plus 21 plus..... plus straight T subscript straight n minus 2 end subscript space plus space straight T subscript straight n minus 1 end subscript plus straight T subscript straight n

Subtracting,     0 space equals space 3 plus left square bracket 4 plus 6 plus 8 plus 10 plus 1..... to space left parenthesis straight n space minus space 1 right parenthesis space terms right square bracket space minus space straight T subscript straight n.

∴                   straight T subscript straight n space equals space 3 plus fraction numerator straight n minus 1 over denominator 2 end fraction. space left parenthesis 2 straight n plus 4 right parenthesis space equals space 3 space plus space left parenthesis straight n minus 1 right parenthesis space left parenthesis straight n plus 2 right parenthesis

or                space space straight T subscript straight n space equals space straight n squared plus straight n plus 1

∴                  straight S subscript straight n space equals space sum from straight k equals 1 to straight n of straight T subscript straight k space equals space sum from straight k equals 1 to straight n of left parenthesis straight k squared plus straight k plus 1 right parenthesis space equals space sum from straight k equals 1 to straight n of straight k squared space plus space sum from straight k equals 1 to straight n of space straight k space plus space sum from straight k equals 1 to straight n of 1

or                  straight S subscript straight n space equals space fraction numerator straight n left parenthesis straight n plus 1 right parenthesis left parenthesis 2 straight n plus 1 right parenthesis over denominator 6 end fraction plus fraction numerator straight n left parenthesis straight n plus 1 right parenthesis over denominator 2 end fraction plus straight n space equals space straight n open square brackets fraction numerator left parenthesis straight n plus 1 right parenthesis left parenthesis 2 straight n plus 1 right parenthesis over denominator 6 end fraction plus fraction numerator left parenthesis straight n plus 1 right parenthesis over denominator 2 end fraction plus 1 close square brackets

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                          equals space space straight n over 6 left square bracket 2 straight n squared plus 6 straight n plus 10 right square bracket space equals space straight n over 3 left parenthesis straight n squared plus 3 straight n plus 5 right parenthesis

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133. The ages of the students in a class form an A.P. whose common difference is 4 months. If the youngest student is 8 years old and the sum of the ages of all the students in the class is 168 years. Find the numbers of students in the class.
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134.

A man deposited Rs. 10,000 in a bank at the rate of 5% simple interest annually. Find the amount in 15th year since he deposited the amount and also calculate the total amount after 20 years.

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135. An insect starts from a point and travels in a straight path 1 mm in the first second and half of the distance covered in the previous second in the succeeding second. In what time would it reach a point 3 mm away from its starting point?
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136. The interior angles of a polygon are in A.P. The smallest angle is 120° and the common difference is 5°. Find the number of sides of the polygon.
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 Multiple Choice QuestionsLong Answer Type

137. Shamshad Ali buys a scooter for Rs 22000. He pays Rs. 4000 cash and agrees to pay the balance in annual instalment of Rs 1000 plus 10% interest on the unpaid amount. How much will the scooter cost him? 
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 Multiple Choice QuestionsShort Answer Type

138. A man buys every year Bank’s cash certificates of value exceeding the last year purchase by Rs. 25. After 20 years, he finds that the total value of the certificates purchased by him is Rs. 7250. Find the values of the certificates purchased by him

(i) In the first year. (ii) In the 13th year.
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 Multiple Choice QuestionsLong Answer Type

139. An odd number of stones lie along a straight path, the distance between any two consecutive stones being 10 m. The stones are to the collected at the place where the middle stone lies. A man can carry only one stone at a time. He starts carrying the stones beginning from the extreme stone. If he covers a path of 3 km, how many stones are there ?
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 Multiple Choice QuestionsShort Answer Type

140. The income of a person is Rs. 3,00,000 in first year and he receives an increase of Rs. 10,000 to his income per year for next 19 years. Find the total amount, he received in 20 years. 
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