In the diagram, ABCD is a square. E is a point on AB such that DE = 10. The angle DEB = x. Find DEC in terms of x.

a) Find $\angle DEC$

Step 1: Identify known angles.
The square symbol at E indicates that $\angle CEB$ is a right angle.

$$\angle CEB = 90^\circ$$

The angle $\angle DEB$ is given as $x$.

Step 2: Use the angle addition postulate.
The angle $\angle CEB$ is composed of $\angle DEC$ and $\angle DEB$.

$$\angle CEB = \angle DEC + \angle DEB$$

Step 3: Substitute known values and solve for $\angle DEC$.

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