
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
Observe the following expressions:
①$1\times 3-2^{2}=3-4=-1$
②$2\times 4-3^{2}=8-9=-1$
③$3\times 5-4^{2}=15-16=-1$
④＿＿＿
$\ldots$
(1) Please write out the fourth expression according to the above pattern;
(2) Express this pattern with a formula containing variables;
(3) Do you think the formula written in (2) must be true? Please explain your reasoning.

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