Let's make a diagram to understand better the question:
Number of degrees of c+d is 180° which leads to the equation d=180-c.
We know that the sum of all intern angles of a triangle must add up 180°, then:
Number of degrees of a+b+c is 180 which leads to the equation a+b=180-c
Then, using the two equations, it can be determined that the value of d is equal to the value of
[tex]\begin{gathered} d=180-c \\ a+b=180-c \\ So, \\ d=a+b \end{gathered}[/tex]
