Find all functions $f: \mathbb{R}\to\mathbb{R}$ satisfying:

$\frac{f(xy)+f(xz)}{2} - f(x)f(yz) \geq \frac{1}{4}$  for all $x,y,z \in \mathbb{R}$
1. Let \( P(x, y, z) \) denote the assertion of the given functional inequality:
   \[
   \frac{f(xy) + f(xz)}{2} - f(x)f(yz) \geq \frac{1}{4}
   \]
   for all \( x, y, z \in \mathbb{R} \).

2. Consider \( P(1, 1, z) \):
   \[
   \frac{f(1 \cdot 1) + f(1 \cdot z)}{2} - f(1)f(1 \cdot z) \geq \frac{1}{4}
   \]
   Simplifying, we get:
   \[
   \frac{f(1) + f(z)}{2} - f(1)f(z) \geq \frac{1}{4}
   \]
   Let \( f(1) = a \). Then:
   \[
   \frac{a + f(z)}{2} - af(z) \geq \frac{1}{4}
   \]
   Rearranging, we obtain:
   \[
   \frac{a + f(z)}{2} - af(z) \geq \frac{1}{4}
   \]
   \[
   a + f(z) - 2af(z) \geq \frac{1}{2}
   \]
   \[
   a + f(z)(1 - 2a) \geq \frac{1}{2}
   \]
   \[
   f(z)(1 - 2a) \geq \frac{1}{2} - a
   \]

3. Consider \( P(x, 1, 1) \):
   \[
   \frac{f(x \cdot 1) + f(x \cdot 1)}{2} - f(x)f(1 \cdot 1) \geq \frac{1}{4}
   \]
   Simplifying, we get:
   \[
   f(x) - f(x)f(1) \geq \frac{1}{4}
   \]
   Let \( f(1) = a \). Then:
   \[
   f(x) - af(x) \geq \frac{1}{4}
   \]
   \[
   f(x)(1 - a) \geq \frac{1}{4}
   \]
   Since \( f(x) \geq \frac{1}{4(1 - a)} \), we need \( 1 - a \neq 0 \), i.e., \( a \neq 1 \).

4. Consider \( P(x, 0, 0) \):
   \[
   \frac{f(x \cdot 0) + f(x \cdot 0)}{2} - f(x)f(0 \cdot 0) \geq \frac{1}{4}
   \]
   Simplifying, we get:
   \[
   f(0) - f(x)f(0) \geq \frac{1}{4}
   \]
   Let \( f(0) = b \). Then:
   \[
   b - bf(x) \geq \frac{1}{4}
   \]
   \[
   b(1 - f(x)) \geq \frac{1}{4}
   \]
   Since \( b \geq \frac{1}{4(1 - f(x))} \), we need \( 1 - f(x) \neq 0 \), i.e., \( f(x) \neq 1 \).

5. From the previous steps, we have:
   \[
   f(x) \geq \frac{1}{2} \quad \text{and} \quad f(x) \leq \frac{1}{2}
   \]
   Therefore, \( f(x) = \frac{1}{2} \) for all \( x \in \mathbb{R} \).

6. Finally, we verify that \( f(x) = \frac{1}{2} \) satisfies the original inequality:
   \[
   \frac{f(xy) + f(xz)}{2} - f(x)f(yz) = \frac{\frac{1}{2} + \frac{1}{2}}{2} - \frac{1}{2} \cdot \frac{1}{2} = \frac{1}{2} - \frac{1}{4} = \frac{1}{4}
   \]
   which is indeed \(\geq \frac{1}{4}\).

Thus, the only function that satisfies the given inequality is \( f(x) = \frac{1}{2} \) for all \( x \in \mathbb{R} \).

The final answer is \( \boxed{f(x) = \frac{1}{2} \text{ for all } x \in \mathbb{R}} \).