To find the value of $س$ in the given triangle, let's analyze the problem based on the image:

We have a triangle with the following angles labeled:

• One angle is $30^\circ$
• Another angle is $120^\circ$
• The third angle is labeled $س$

Since the sum of the internal angles in any triangle is always $180^\circ$, we can set up the following equation:

$30 + 120 + س = 180$

Simplifying this:

$150 + س = 180$

Subtracting $150$ from both sides:

$س = 180 - 150$ $س = 30$

Thus, the value of $س$ is $30^\circ$.

Would you like further details or have any additional questions?

Here are some related questions to expand your understanding:

1. What is the formula for finding the missing angle in a triangle?
2. How do you solve for an unknown angle in an isosceles triangle?
3. What are the properties of a triangle with two equal angles?
4. How does the angle sum property help in solving for unknown angles in polygons?
5. What are the different types of triangles based on angles?

Tip: Always remember that the sum of interior angles in any triangle is $180^\circ$, which is useful for solving unknown angles.