Question

find the sum of 20 terms of an A.P.if its first term is 2 and the seventh term is 0​

Answer 1

a=2 , a7=0

an=a+(n-1)d
0=2+(7-1)d
-2=6d
d=-1/3

Sn=n/2[2a+(n-1)d]
S20=20/2[4+(19)(-1/3)]
S20=10(12-19/3)
S20=10(-7/3)
S20= -70/3

Answer 2

Answer:

-23.3 is the answer.

Hope this help.

Step-by-step explanation:

first term of AP (a) =2

seventh term of AP (a7)= 0

Using formula ----- (a)n = a+(n-1)d

a(7) =a+(7-1)d

a(7)=a +6d..........(I)

put value of a=2 in equation number .....(I)

a(7)=a+6d

a(7)= 2+6d

0= 2+6d {a(7)=0}

-2=6d

-2/6=d

d= -1/3

now , using formula...S(n)= n/2[2a+(n-1)d

S(20)=20/2[2×2+(20-1)×-1/3]

S(20)=10[4+19×-1/3]

S(20)=10[4-19/3]

S(20)=10[4-6.33]

S(20)=10[-2.33]

S(20)= -23.3 .......is the sum of 20 terms of the AP