Question

a5=40 , a10=20 then a15=? ​

Answer 1

Step-by-step explanation:

Given,

a5=40 , a10=20

then a15=?

We have,

a5= 40

a+4d= 40

a= 40-4d..............(1)

Also,

a10=20

a+9d= 20

a= 20-9d...............(2)

From (1) and (2)

40-4d = 20-9d

or, 40-20 = -9d+4d

or, -20 = -5d

or, 20/5 = d

or, d= 4...........(3)

Using the value of "d" from (3) in (1)

(1) implies,

a = 40-4d

a = 40- 4×4

a = 40-16

a= 24...................(4)

Hence,

a15 = a+14d

a15 = 24+ 14×4 [Using, (3) and (4)]

a15 = 24+56

a15 = 80 ANS

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