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Answer 1

Hope it helps.
Will be easy to understand.

Answer 2

Let the first term be x and common difference be y

Sp = ap^2 + bp

=> p/2 [ 2x + (p - 1)y] = p( ap + b)

=> 1/2 [ 2x + py - y] = ap + b

=> 2x + py - y = 2ap + 2b

=> y(p-1) + 2x = 2ap - 2a + 2b + 2a [ add and subtract 2a]

=> 2x + (p - 1)y = 2a + 2b + 2ap - 2a

=> 2x + (p - 1) y = 2(a+b) + 2a( p-1)

On Comparing both sides, we get

2x = 2(a +b)

=> x = a+b

First term = a +b


And

y = 2a

Common difference = 2a