Q18 of 167 Page 20

Mark the correct alternative in the following:

If the first, second and last term of an A.P. are a, b and 2a respectively, then its sum is


Here,a1=a;a2=b and l=2a


Then d=a2-a1=b-a


So,l=an=a1+(n-1)d


∴ 2a=a+(n-1)(b-a)


∴ a=n(b-a)-(b-a)


∴ a=n(b-a)-b+a


∴ n(b-a)=b



Now,


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