In an APGiven a = 5, d = 3, an =50, find n and Sn. from Mathem

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 Multiple Choice QuestionsShort Answer Type

61. Find the sums given below:
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62. Find the sums given below:
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63.

In an AP
Given a = 5, d = 3, a=50, find n and Sn.


Here a = 5, d = 3, an = 50
We know that, an = a + (n – 1 )d
⇒ 50 = 5 + (n – 1) x 3
⇒ 50 – 5 = 3 x (n – 1)
⇒ 45 = 3 x (n – 1)
rightwards double arrow space space straight n space minus space 1 space equals space 45 over 3 equals 15
rightwards double arrow space space straight n space equals space 15 space plus space 1 space equals space 16
And comma space space space space space straight S subscript straight n space equals space straight n over 2 left square bracket 2 straight a space plus space left parenthesis straight n minus 1 right parenthesis straight d right square bracket
therefore space space space straight S subscript 16 equals 16 over 2 left square bracket 2 cross times 5 plus left parenthesis 16 minus 1 right parenthesis cross times 3 right square bracket
space
      = 8 x [ 10 + 15 x 3]
      = 8 x 10 + 45
      = 8 x 55 = 440
Hence,   n = 16
and       S=S16 = 440

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64.

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65.

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66. Given a3 = 75, S10 = 120, find ‘d’ and a10.
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67.

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68. Find the sum of the series 5 + 7 + 9+ 10 + 13 + 13 + 17 + 16 +.........to 40 terms.
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70. Prove that no matter what the real numbers ‘a’ and ‘b’ are the sequence with nth term a + nb is always an A .P. what is the common difference ? What is the sum of the first 20 terms.
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