Question

If 2nd and 4th term of AP are 12 and 20 respectively find sum of first 25 term​

Answer 1

Explanation:

a2=a+(2-1)d

12=a+d------(i)

a4=a+(4-1)d

20=a+3d-------(ii)

Subtracting (i) and (ii)

8=2d

d=4

12=a+d

a=12-d

a=12-4

a=8

S=25/2[2x8+(25-1)4]

S=25/2[16+96]

S=12.5x112

S=1400