Answer:
(a) 720 ways
(b) 120 ways
(c) 24 ways
Step-by-step explanation:
Given

--- number of letters
Solving (a): Number of arrangements.
We have:

So, the number of arrangements is calculated as:

This gives:

This gives:


Solving (b): DA as a unit
DA as a unit implies that, we have:
[DA] N C E R
So, we have:

So, the number of arrangements is calculated as:

This gives:

This gives:


Solving (c): NCE as a unit
NCE as a unit implies that, we have:
D A [NCE] R
So, we have:

So, the number of arrangements is calculated as:

This gives:

This gives:


Let

where we assume |r| < 1. Multiplying on both sides by r gives

and subtracting this from
gives

As n → ∞, the exponential term will converge to 0, and the partial sums
will converge to

Now, we're given


We must have |r| < 1 since both sums converge, so


Solving for r by substitution, we have


Recalling the difference of squares identity, we have

We've already confirmed r ≠ 1, so we can simplify this to

It follows that

and so the sum we want is

which doesn't appear to be either of the given answer choices. Are you sure there isn't a typo somewhere?
Answer:I am pretty sure it is A
Step-by-step explanation:
Use the following identities:

Also because the angle is in quadrant 3, sin must be negative.
Therefore

Subbing in tan = 0.958