the sum of first n terms of an ap is 5n×n+3n,if its mth term is 168,find the value of m.also,find 20th term of this ap.
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put n=1 we get first term =8 say a
put n=2 we will get sum=26 .subtract 8 from it we will get second term as 18
so common difference (d)=10
now mth term=a+(m-1)d
168=8+(m-1)10
m=17
20th term a+19d=8+190=198
put n=2 we will get sum=26 .subtract 8 from it we will get second term as 18
so common difference (d)=10
now mth term=a+(m-1)d
168=8+(m-1)10
m=17
20th term a+19d=8+190=198
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