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| 1. |
If an AP has a=1,tn=20,sn=399 then value of n is |
| Answer» 20=1+(n-1)d20-1=(n-1)d19=(n-1)d--------------------eq1399=n/2(2+(n-1)d)from eq1399=n/2(2+19)399/21=n/219=n/219/2=n9.5=n | |