Question

how did we you multiply 2a+9d=25 and a+ 2d= 15

Answer 1

Step-by-step explanation:

Given that,

a3 = 15,

S10 = 125

As an = a + (n − 1)d,

a3 = a + (3 − 1)d

15 = a + 2d ... (i)

Sn = n/2 [2a + (n - 1)d]

S10 = 10/2 [2a + (10 - 1)d]

125 = 5(2a + 9d)

25 = 2a + 9d ... (ii)

On multiplying equation (i) by (ii),

we get ,

30 = 2a + 4d ... (iii)

On subtracting equation (iii) from (ii),

we get,

−5 = 5d d = −1

From equation (i),

15 = a + 2(−1)

15 = a − 2

a = 17

a10 = a + (10 − 1)d

a10 = 17 + (9) (−1)

a10 = 17 − 9 = 8

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