Question

The sum of first 9 terms is 81. Sum of first 15 terms is 225. Find a and d​

Answer 1

Answer:

d=2

a =1

Step-by-step explanation:

sn = n/2( 2a + (n-1)d)

81 = 9/2 (2a +8d)---1

225 =15/2 (2a+14d)----2

solve 1 & 2 we get

d =2

a=1

i hope u got it

Answer 2

Answer:

a = 1 and d = 2

Step-by-step explanation:

Sum of first 9 terms = 81

Sum = n/2(2a + (n-1)d)

81 = 9/2(2a + 8d)

9 = 1/2 * 2(a+4d)

9 = a + 4d ........(i)

Sum of first 15 terms = 225

225 = 15/2(2a + 14d)

15 = 1/2 * 2(a+7d)

15 = a + 7d ..........(ii)

Subtracting (i) from (ii) --

15 = a + 7d

09 = a + 4d

(-) (-) (-)

-----------------

6 = 3d

d = 2

Putting d = 2 in (i),

we get -

a = 1