To find the value of 8ab, we first need to expand and simplify the given expressions.

Given:
(a+b)² = a² + 2ab + b²

Therefore, (a+b)² = a² + 2ab + b² = 25

Similarly,
(a-b)² = a² - 2ab + b² = 8

Now, let's find the value of 8ab:

From the given expressions, we have:
a² + 2ab + b² = 25
a² - 2ab + b² = 8

Adding the above two equations, we get:
2a² + 2b² = 33

Dividing by 2 on both sides, we get:
a² + b² = 16.5

Now, squaring this equation, we get:
(a² + b²)² = (16.5)²
a⁴ + 2a²b² + b⁴ = 272.25

Substitute the value of a⁴ + b⁴ = (a² + b²)² - 2a²b²
(16.5)² - 2a²b² = 272.25
272.25 - 2a²b² = 272.25

Solving the above equation, we get:
2a²b² = 0
a²b² = 0

Therefore, the value of 8ab is 0.