Q : 17      Find the \small 20th term from the last term of the AP :  \small 3,8,13,...,253.

Answers (1)

Given AP is 
\small 3,8,13,...,253
Here, a = 3 \ and \ a_n = 253
And
d = a_2-a_1=8-3 = 5
Let suppose there are n terms in the AP
Now, we know that
a_n = a+(n-1)d
253= 3+(n-1)5
5n = 255
n = 51
So, there are 51 terms in the given AP and 20th term from the last  will be 32th term from the starting
Therefore,
a_{32} = a+31d
a_{32} = 3+31\times 5 = 3+155 = 158
Therefore, 20th term from the last of given AP is 158

 

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