Question

the sum of first three terms of an ap is 15 the sum of next three term is 42 then the sum of first 9 terms of the ap is​

Answer 1

Answer:

126

Step-by-step explanation:

first three terms sum = 15

that is a+a+d+a+2d =15

3a + 3d = 15

a+d = 15------------@

also next 3 terms sum =42

that is a+3d+a+4d+a+5d = 42

3a+12d= 42

a+4d= 14------------@

subtracting both the equation

a=2 and d=3

so sum of nine terms Sn = n/2[2a + (n-1)d

put n=9

so Sn = 126

Answer 2

Step-by-step explanation:

Answer:

126

Step-by-step explanation:

first three terms sum = 15

that is a+a+d+a+2d =15

3a + 3d = 15

a+d = 15------------@

also next 3 terms sum =42

that is a+3d+a+4d+a+5d = 42

3a+12d= 42

a+4d= 14------------@

subtracting both the equation

a=2 and d=3

so sum of nine terms Sn = n/2[2a + (n-1)d

put n=9

so Sn = 126