using the fact that 5pi/8=1/2(5pi/8) and the half angle identity for cos to find the exact (in terms of radicals) value of cos(5pi/8)

To find the exact value of cos(5π/8) using the half angle identity for cosine, we can start by rewriting 5π/8 as 1/2(5π/8).

Using the half angle identity for cosine, we have:

cos(θ/2) = ± √((1 + cos(θ))/2)

Here, θ = 5π/8.

So, cos(5π/8) can be expressed as cos((1/2)(5π/8)).

Using the half angle identity for cosine, we get:

cos(5π/8) = ± √((1 + cos(5π/4))/2)

Now, let's find the value of cos(5π/4):

Since 5π/4 is in the third quadrant, where cosine is negative, we can express it as cos(5π/4) = -√2/2.

Substituting this value into the expression for cos(5π/8):

cos(5π/8) = ± √((1 - √2/2)/2)

Now, let's simplify this expression:

cos(5π/8) = ± √((2 - √2)/4)

Simplifying further, we have:

cos(5π/8) = ± √(2 - √2)/2)

Thus, the exact value of cos(5π/8) in terms of radicals is ± √(2 - √2)/2.

To find the exact value of cos(5π/8) using the half angle identity, we start by rewriting 5π/8 as the product of 1/2 and 5π/8:

5π/8 = (1/2)(5π/8)

Now, we can use the half angle identity for cosine, which states that:

cos(θ/2) = ±√[(1 + cos(θ))/2]

Let's apply this identity to our expression:

cos(5π/8) = cos((1/2)(5π/8))

Now, using the half angle identity:

cos((1/2)(5π/8)) = ±√[(1 + cos(5π/8))/2]

To determine the sign (±), we need to consider the quadrant where 5π/8 lies.

In the first quadrant, the value of cos(θ) is positive.
In the second quadrant, the value of cos(θ) is negative.

Since 5π/8 is between π/2 and π, it lies in the second quadrant, and therefore cos(5π/8) is negative.

Now, let's plug the value of cos(5π/8) into the half angle identity equation:

cos((1/2)(5π/8)) = -√[(1 + cos(5π/8))/2]

We still need to find the value of cos(5π/8), which we can obtain using the half angle identity again.

Let's define a new angle, let's say α, which satisfies the equation:

(1/2)(5π/8) = α/2

Simplifying the equation, we get:

5π/8 = α

Now, we can rewrite cos((1/2)(5π/8)) in terms of α:

cos((1/2)(5π/8)) = cos(α/2)

Applying the half angle identity to α/2, we have:

cos(α/2) = ±√[(1 + cos(α))/2]

Since α = 5π/8, we can substitute it back:

cos(5π/8) = ±√[(1 + cos(5π/8))/2]

We now have an equation involving cos(5π/8) and we can solve it.

To do so, let's multiply both sides by 2:

2cos(5π/8) = ±√(1 + cos(5π/8))

Next, square both sides to eliminate the radical:

(2cos(5π/8))^2 = (±√(1 + cos(5π/8)))^2

4cos^2(5π/8) = 1 + cos(5π/8)

Now, let's rearrange the equation:

4cos^2(5π/8) - cos(5π/8) - 1 = 0

This is a quadratic equation in terms of cos(5π/8). We can solve it by factoring or using the quadratic formula:

cos(5π/8) = [-(-1) ± √((-1)^2 -4(4)(-1))]/(2(4))

cos(5π/8) = (1 ± √(1 + 16))/8

cos(5π/8) = (1 ± √17)/8

Therefore, the exact value of cos(5π/8) in terms of radicals is:

cos(5π/8) = (1 ± √17)/8