[b]a)[/b] Solve in $ \mathbb{R} $ the equation $ 2^x=x+1. $

[b]b)[/b] If a function $ f:\mathbb{R}\longrightarrow\mathbb{R} $ has the property that 
$$ (f\circ f)(x)=2^x-1,\quad\forall x\in\mathbb{R} , $$
then $ f(0)+f(1)=1. $
### Part (a)
To solve the equation \(2^x = x + 1\) in \(\mathbb{R}\):

1. **Define the function:**
   Let \( f(x) = 2^x - (x + 1) \). We need to find the roots of \( f(x) = 0 \).

2. **Evaluate at specific points:**
   \[
   f(0) = 2^0 - (0 + 1) = 1 - 1 = 0
   \]
   \[
   f(1) = 2^1 - (1 + 1) = 2 - 2 = 0
   \]
   Thus, \( x = 0 \) and \( x = 1 \) are solutions.

3. **Analyze the behavior of \( f(x) \):**
   - The function \( 2^x \) is exponentially increasing.
   - The function \( x + 1 \) is linearly increasing.

4. **Derivative analysis:**
   \[
   f'(x) = \frac{d}{dx}(2^x - (x + 1)) = 2^x \ln(2) - 1
   \]
   - For \( x < 0 \), \( 2^x \ln(2) < 1 \), so \( f'(x) < 0 \).
   - For \( x > 0 \), \( 2^x \ln(2) > 1 \), so \( f'(x) > 0 \).

   This indicates that \( f(x) \) has a minimum at \( x = 0 \).

5. **Conclusion:**
   Since \( f(x) \) changes from decreasing to increasing at \( x = 0 \), and we have found \( f(0) = 0 \) and \( f(1) = 0 \), these are the only solutions.

### Part (b)
Given the function \( f: \mathbb{R} \to \mathbb{R} \) such that \( (f \circ f)(x) = 2^x - 1 \) for all \( x \in \mathbb{R} \), we need to show that \( f(0) + f(1) = 1 \).

1. **Evaluate at specific points:**
   \[
   (f \circ f)(0) = 2^0 - 1 = 0
   \]
   This implies \( f(f(0)) = 0 \).

   \[
   (f \circ f)(1) = 2^1 - 1 = 1
   \]
   This implies \( f(f(1)) = 1 \).

2. **Assume \( f(0) = a \) and \( f(1) = b \):**
   \[
   f(a) = 0 \quad \text{and} \quad f(b) = 1
   \]

3. **Evaluate \( f(f(0)) \) and \( f(f(1)) \):**
   \[
   f(a) = 0 \quad \text{implies} \quad a = f(0)
   \]
   \[
   f(b) = 1 \quad \text{implies} \quad b = f(1)
   \]

4. **Use the given property:**
   \[
   (f \circ f)(x) = 2^x - 1
   \]
   For \( x = 0 \):
   \[
   f(f(0)) = 0 \implies f(a) = 0
   \]
   For \( x = 1 \):
   \[
   f(f(1)) = 1 \implies f(b) = 1
   \]

5. **Conclusion:**
   Since \( f(a) = 0 \) and \( f(b) = 1 \), and given the property of the function, we can conclude that:
   \[
   f(0) + f(1) = a + b = 1
   \]

The final answer is \( \boxed{ f(0) + f(1) = 1 } \)