
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
In $\triangle ABC$, the sides opposite to angles $A$, $B$, $C$ are respectively $a$, $b$, $c$. It is given that $\dfrac{b}{a+c} = \dfrac{a+b-c}{a+b}$.

(Ⅰ) Find the angle $A$.

(Ⅱ) If $a=15$ and $b=10$, find the value of $\cos B$.

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