Question

How many terms if an ap9,17,25......... must be taken so that is 636?

Answer 1

question is wrong

plz check it nd then resend it

Answer 2

Let the last term be x

So an=a+(n-1)d

a=9

d=a2-a1=17-9

=8

n be y

So putting the values,

an=9+(y-1)8

an=9+8y-8

x =8y+1

Sn=n/2(a+L)

[We know that L=an]

Sn is given as 636

Therefore,

636=y/2(9+x)

[x=8y+1]

636=y/2(9+8y+1)

636=y/2(10+8y)

1272=10y+8y2 (8y square)

The equation comes out to be

8y2+10y-1272=0

Taking 2 as common from both sides we get

4y2+5y-636=0

Now by solving this quadratic equation we will get two values of y that Is,

12 and -53/4 we can't take n -be

So y=n=12

Therefore number of terms must be 12