Question

4. t14 is twice its t8 , if t6 is -8 find S20

Answer 1

Answer:

S20 = -280

Step-by-step explanation:

Given :

t14= 2( t8)

=> a+13d = 2(a+7d)

=> a+13d = 2a+14d

=> a-d = 0 -----------> eq 1

Also given,

t6= a+5d = -8 ------------> eq 2

eq 2 - eq 1

=> 6d = -8

=> d = -4/3

Substituting the value of d in eq 1

=> a + 4/3 = 0

=> a = -4/3

Sn = n/ 2 [2a+(n-1) d]

S20 = 20/2 [ 2(-4/3) + 19(-4/3)

= 10( -8/3 - 76/3)

= 10( - 84/3)

= 10(-28) = -280

Hope it helps......!