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in Arithmetic Progression by (56.3k points)

The nth term of an A.P is given by (-4n + 15). Find the sum of first 20 terms of this A.P.

1 Answer

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Best answer

Given,

The nth term of the A.P = (-4n + 15)

So, by putting n = 1 and n = 20 we can find the first ans 20th term of the A.P

a = (-4(1) + 15) = 11

And,

a20 = (-4(20) + 15) = -65

Now, for find the sum of 20 terms of this A.P we have the first and last term.

So, using the formula

Sn = \(\frac{n}{2}\)(a + l)

S20  = \(\frac{20}{2}\)(11 + (-65))

= 10(-54)

= -540

Therefore, the sum of first 20 terms of this A.P. is -540.

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