Schoolcrop

(Q17). Find the sums given below.

(ii) 34 + 32 + 30 + … + 10

Given A.P: 34 + 32 + 30 + … + 10

a = ; d = a₂ – a₁ = – = - and the last term l = a_n =

But, a_n = a + (n – 1) d

∴ = + (n – 1) (-)

⇒ – = -n +

⇒ -n = - –

⇒ n =

-

-

=

∴ n =

Also, S_n =

n

2

(a + l)

where a = ; l =

S₁₃ =

2

( + )

=

2

×

= ×

=