Given,

, we need to find value of n

So we will first find the limit and then equate it with 405 to get the value of n.

We need to find the limit for:

As limit can’t be find out simply by putting x = a because it is taking indeterminate form(0/0) form, so we need to have a different approach.

Let, Z =

Note: To solve the problems of limit similar to one in our question we use the formula mentioned below which can be derived using binomial theorem.

Formula to be used:

As Z matches exactly with the form as described above so we don’t need to do any manipulations–

Z =

Use the formula:

∴ Z = 5(a)^5–1 = 5a⁴

According to question Z = 405

∴ 5(a)⁴ = 405

⇒ a⁴ = 81 = 3⁴ or (–3)⁴

Clearly on comparison we have –

a = 3 or –3