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

1.	The nth term of an A.P is given by (-4n + 15). Find the sum of first 20 terms of this A.P.
Answer» Given, The n^th term of the A.P = (-4n + 15) So, by putting n = 1 and n = 20 we can find the first ans 20^th 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 S_n = \(\frac{n}{2}\)(a + l) S₂₀ = \(\frac{20}{2}\)(11 + (-65)) = 10(-54) = -540 Therefore, the sum of first 20 terms of this A.P. is -540.

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

Answer»

Given,

The n^th term of the A.P = (-4n + 15)

So, by putting n = 1 and n = 20 we can find the first ans 20^th 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

S_n = \(\frac{n}{2}\)(a + l)

S₂₀ = \(\frac{20}{2}\)(11 + (-65))

= 10(-54)

= -540

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