Question

Find the sum of the first 20 terms of an a.p whose 3rd term is 5 and 4th term is 12.

Answer 1

Verify the answer from the given attachment

Answer 2

Answer:

Step-by-step explanation: let a be the first term and d is common difference.

3rd term is 5 and 4th term is 12 .

tn = a + ( n-1) d

t3=a+(3-1)d

t3= a+2d

5=a+2d ......1

t4=a+3d

12=a+3d ......2

Subtract equation 1 from 2.

12-5=a+ 3d - a + 2d

7=d

Put d in equation 1 or 2

You get

a = -9.

S20=?

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

S20= 20/2 [2 (-9) + (20-1) 7]

S20 = 1150