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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