Very easy!!!!!!!!

 

Take the first term to be a and the last term to be b. Let the common difference be  d .

then, mulitiplying 1st and last you get: ab

and multiplying 2nd and 2nd last you get ab+d(b-a-d).

Now, if d>0, b>a and also b> a+d

so, b-a-d >0

and hence d(b-a-d) > 0

If d<0

b<a

and b<a+d,

so, b-a-d<0

so d(b-a-d)>0

so, product of 2nd and 2nd last = ab+some positive number and is hence greater than ab[product of 1st and last]

So,

ab < ab+d(b-a-d)

so, ab+d(b-a-d)< ab + 2d(b-a-d)

i.e. product of 2nd and 2nd last< product of 3rd and 3rd last.

IN this way the product of equidistant terms keep on incresing as one moves closer to the middle term.

 

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