A number theory problem by Aditya Raj

Number Theory Level 1

Find the 20th term from the end of the AP 3, 8, 13……..253.

157 133 185 158

Aditya Raj by Aditya Raj , 22, India

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7 solutions

Amritesh Anand
Apr 19, 2014

ncert 10th standard exercise 5.2

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no bro i study in icse board &it is ques from class 11th

Aditya Raj - 7 years, 1 month ago

Hello,this is quite simple...

as for an arithmetic progression, 3,8,13......253,

by using an arithmetic formula,

253 = 3 + (n-1)5

253 = 3 + 5n -5

255 / 5 = n

n = 51, there are 51 terms in total,

as for the 20th term from the last 1,

= 51 -20 + 1 = 32,

T(32) = a + 31d

T(32) = 3 + 31(5) = 158

Thanks...

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Jo E
Apr 3, 2014

l - (20 - 1) x 5 = 253- 19 X 5 = 253 - 95 = 158.

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Bhavesh Bhagde
Mar 29, 2014

x=a +(n-1)d , then x=158

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Himanshu Dave
Mar 22, 2014

Here, We have a = 253, d = -5, and n = 20 X=Desired number now applying the formula: X = a + (n-1)d; we get, X = 253 + (20-1) (-5); X = 253 + (19) (-5); X = 253 - 95; X = 158

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Niket Patel
Mar 13, 2014

253= 3 +(n-1) 5 ------>>>> n=51 20th from last --------- >>> 51-20+1 =32 ans = 3+(32-20) 5------------>>> ans 158

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(y) but why not just make the 'd' = -5?

Himanshu Dave - 7 years, 2 months ago
Meldrin Rebello
Mar 9, 2014

158

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