
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
Three lines $l_1$: $2x-y-10=0$, $l_2$: $4x+3y-10=0$, $l_3$: $ax+2y-8=0$  
(1) Find the angle between $l_1$ and $l_2$. (Express the answer using an inverse trigonometric function)  
(2) If the three lines $l_1$, $l_2$, $l_3$ cannot form a triangle, find all possible values of $a$.

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