
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
Which of the following statements is correct:

1. The vectors $\overrightarrow {a}=(2,-3)$ and $\overrightarrow {b}=( \frac {1}{2},- \frac {3}{4})$ can be used as a basis for all vectors in the plane.
2. If A > B, then sinA > sinB.
3. In △ABC, if a=5, b=8, c=7, then $\overrightarrow {BC} \cdot \overrightarrow {CA} = 20$.
4. Given the sequence {a<sub>n</sub>} that satisfies a<sub>1</sub> = 1 and a<sub>n</sub> - a<sub>n-1</sub> = n, then a<sub>n</sub> = $\frac {n(n+1)}{2}$.
5. If acosB - bcosA = c, then △ABC is definitely an isosceles right triangle.

Correct sequence number: \_\_\_\_\_\_

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