Question

if an ap if the 12th term is -13 and the sum of the fourth term is 24 find the sum of the first 20 terms​

Answer 1

Step-by-step explanation:

A12 = -13

S4 = 24

A12 = a+(n-1 )d

A12=A+11d

-13=A+11d. -------(1)

S4 =A1+A2+A3+A4

24=A+A+d+A+2d+A+3d

24= 4A+6d

12=2A+3d. ----------(2)

By Elimination Method

equation 1 multiply by 2

2A+22d=-26

2A+3d. =12

- - -

19d=-38

d=-2

Then a+11(2)=-13

a+22=-13

a= -13-22

a= -35

S20 = 10(2(-35)+19*-2

S20 = 10(-70-38)

S20 = 10(-108)

S20 = -1080

to appreciate me my efforts please leave a thanks