
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
The following are five propositions:
1. In a plane, fixed points $F_{1}$ and $F_{2}$ have a distance of $|F_{1}F_{2}| = 6$. If a moving point $M$ satisfies $|MF_{1}| + |MF_{2}| = 6$, then the locus of point $M$ is an ellipse.
2. "In $\triangle ABC$ with $\angle B = 60°$" is a necessary and sufficient condition for "$\angle A$, $\angle B$, and $\angle C$ are an arithmetic progression."
3. "x = 0" is a sufficient but not necessary condition for "x ≥ 0".
4. Given vectors $\overrightarrow{a}$, $\overrightarrow{b}$, $\overrightarrow{c}$ are a basis in a space, then vectors $\overrightarrow{a} + \overrightarrow{b}$, $\overrightarrow{a} - \overrightarrow{b}$, $\overrightarrow{c}$ also form a basis in that space.
5. For lines $l_{1}: ax + 3y - 1 = 0$ and $l_{2}: x + by + 1 = 0$, a necessary and sufficient condition for $l_{1} \perp l_{2}$ is $\frac{a}{b} = -3$.
The sequence numbers of the true propositions are ______.

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