FIND THE TERM

Number Theory Level 2

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


The answer is 158.

Aditya Raj by Aditya Raj , 22, India

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4 solutions

Isabella Amabel
Apr 20, 2014

Let the last term be l. Here, a=253; d=5; n=20. Thus l = a-(n-1)d = 253 - 19*5 =253-95 = 158

1 Helpful 0 Interesting 0 Brilliant 0 Confused
Nikhil Kashyap
Mar 17, 2014

n − r + 1 = 51 − 20 + 1 = 32 n d t e r m f r o m b e g i n n i n g A 32 = 3 + ( 32 − 1 ) × 5 = 158 n-r+1=51-20+1={ 32 }^{ nd }\quad term\quad from\quad beginning\\ { A }_{ 32 }=3+(32-1)\times 5=\boxed { 158 }

1 Helpful 0 Interesting 0 Brilliant 0 Confused

Then by your reasoning the 1st term from the end = 51 - 1 + 1 = 51st term from the beginning. Therefore A51 = 3 + (51 - 1) x 5 = 253. Something doesn't compute here! The first term from the end is 248, isn't it??

Guiseppi Butel - 7 years, 2 months ago
Rahul Gautam
Apr 6, 2014

t20 = 253 + (20-1)*(-5) = 253 - 95 = 158

0 Helpful 0 Interesting 0 Brilliant 0 Confused

Then by your reasoning the 1st term from the end = 51 - 1 + 1 = 51st term from the beginning. Therefore A51 = 3 + (51 - 1) x 5 = 253. Something doesn't compute here! The first term from the end is 248, isn't it??

Guiseppi Butel - 7 years, 2 months ago
Ali Qaisrani
Mar 26, 2014

very simple its 158

0 Helpful 0 Interesting 0 Brilliant 0 Confused

Then by your reasoning the 1st term from the end = 51 - 1 + 1 = 51st term from the beginning. Therefore A51 = 3 + (51 - 1) x 5 = 253. Something doesn't compute here! The first term from the end is 248, isn't it??

Guiseppi Butel - 7 years, 2 months ago

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