Question

Common difference of an A.P. is -2 and first term is 80. Find the sum if last term is 10.
(a) 1600 (b) 1620 (c) 1650 (d) 1700

Answer 1

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

Step-by-step explanation:

Answer 2

Given /

d=-2

a=80

a(l)=10

Explaination Formula /

an=a+(n-1)d

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

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

70-2=2n

72=2n

72/2=n

n=36

then Sn=??

=>Sn=n/2(a+an)

=36/2(80+10)

=18(90)

=18×90

=1620

=>Sn=1620