Question

The sum of first,third,and seventh term of an ap is 216.Find the sum of the first 13term

Answer 1

t1=a+ d

t3=a+2d

t7=a+6d

t1 +t3 +t7=216

a+d+a+2d+a+6d= 216

3a+9d=216

a+ 3d=72...(1)

t13=a + 12d

so, from (1 )we get

t13 is 4 times of the sum of 1st 3rd nd 7th term

therefore,

a+12d =72×4

, =288