
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 circle $O$: $x^2+y^2=4$ and point $M(1,a)$.

(1) If there is exactly one line passing through point $M$ that is tangent to circle $O$, find the value of the real number $a$.
(2) When $a=2$, find the equation of the tangent line(s) to circle $O$ that pass through point $M$.

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