Question

Common difference of an A.P. is -2 and first term is 80. Find the sum if last term is 10.​

Answer 1

Answer:26 terms

Step-by-step explanation:Here, a = 5, d = 3, an = 80

Let the number of terms be n.

Then, we have:

an = a+n-1d⇒80 = 5+n-13⇒75 = n-13⇒25 =  n-1⇒26 = n

Thus, there are 26 terms in the given A.P.

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Answer 2

Answer:

if d = -2, a = 80, a(l) = 10

Explaintion

an = a+(n-1)d

10 = 80 + ( n-1) (-2)

10-80= -2n + 2

-70-2 = -2n

-72= -2n

72/2= n

36 = n

now, if n= 36

then,Sn= ?

Sn= n/2(a + an)

Sn = 36/2(80+10)

Sn = 18(90)

Sn = 1620