If first term of

We know, sum of ‘n’ terms of an AP

Where a and a_n are first and last term respectively.

Here, a = 5

n = 10

a_n = 45

Putting values, we get

OR

We know that nth term of an A.P is given by,

a_n = a + (n – 1)d

where, a = first term, n = number of terms, and d = common difference.

Therefore,

First term = a

Fourth term, a₄ = a + (4 – 1)d

a₄ = a + 3d

According to question,

a = 2(a + 3d)

a = 2a + 6d

a = -6d