Question

find the 20th term from the last term of the AP:3,8,13,.......,253.

Answer 1

First we can find that which term is 253
We can see that D=5 and a=3
=> 253 = 3 + 5(n-1)
=> 250 = 5(n-1)
=> 50 = n-1
=> 51= n
So 253 is the 51st term
20th term from last = 51-20= 31
31st term = 3 + 5(31-1) = 3 + 150 = 153

Answer 2

Answer:

Un = a + (n-1) d

In this question

a = 253

d = -5 (common difference)( from last)

Finally, 20th term

U20 = 253 +( 20-1)×(-5)

= 253 -100 +5

= 258.

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