
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given vectors $\overrightarrow {a}$=（cos $\frac {x}{2}$, sin $\frac {x}{2}$） and $\overrightarrow {b}$=（cos $\frac {3x}{2}$, -sin $\frac {3x}{2}$）, where x∈[ $\frac {π}{2}$, π].
(I) Find the expression for $\overrightarrow {a}$•$\overrightarrow {b}$ and the range of values for |$\overrightarrow {a}$+$\overrightarrow {b}$|.
(II) Let f(x) = $\overrightarrow {a}$•$\overrightarrow {b}$ - 2λ|$\overrightarrow {a}$+$\overrightarrow {b}$|. If the minimum value of f(x) is -$\frac {3}{2}$, find the value of the real number λ.

Present the answer in LaTex format: \boxed{Your answer}
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