10. In the diagram, mZECA = 120. m2BCD =
.A
(с
D
90
d
180
O
60
о
120

120

Step-by-step explanation:

ECA and ACB must complement each other to 180 degrees.

as must ECD and BCD.

and that is just to start with.

overall this also means that angles at the top of the intersected line must be the same as the angles at the bottom of the intersected line. they are just mirrored, so, left is right and right is left.

therefore BCD = ECA = 120 degrees.

just imagine what would have to happen, so that that relationship between the angles can be destroyed.

right, at least one of the lines has to bend or kink away from the straight line at the intersection point.

but then, these parts of the "lines" are no longer part of the same straight line.

so, for truly straight lines, the mentioned relations between the angles always applies.

step-by-step explanation: