Question

find the sum of indicated number of term of each of the following A.P'.s: 1) 5,2,-1,-4,-7,...;n terms​

Answer 1

Step-by-step explanation:

i) given AP = 16, 11, 6........

here a = 16

d = -5

n = 23

Sn = n/2 { 2a + ( n - 1 ) d }

= 23/2 { 2× 16 + ( 22 × (-5) ) }

= 11.5 ( 32 - 120 )

= 11.5 × (- 78 )

= - 897

ii) given AP = -0.5, -1.0, -1.5........

here a = - 0.5

d = - 0.5

n = 10

Sn = 10/2 { 2×(-0.5) + 9 × (-0.5) }

= 5 { -1 - 4.5 )

= 5 ( -5.5 )

= - 27.5

iii) given AP = -1, 1/4, 3/2.........

here a = -1

d = 5/4

n = 10

Sn = 10/2 { 2 × ( -1 ) + 9 × 5/4 }

= 5 { -2 + 11.25 }

= 5 × 9.25

= 46.25