Quadrilateral relation – right angle conclusion: In quadrilateral ABCD, ∠B = 90° and AD^2 = AB^2 + BC^2 + CD^2. What is the measure of ∠ACD?

Introduction / Context:
This identity resembles Pythagorean-type relations extended to quadrilaterals. With ∠B = 90°, the condition AD^2 = AB^2 + BC^2 + CD^2 signals a right-angle conclusion at C against diagonal AC.

Given Data / Assumptions:

• ABCD is any quadrilateral with ∠B = 90°.
• AD^2 = AB^2 + BC^2 + CD^2.
• We must find ∠ACD.

Concept / Approach:
Consider triangles about diagonal AC. Using the Cosine Rule in ΔABC (right at B) and ΔACD, and comparing with the given sum-of-squares identity, one deduces that the angle at C with respect to AC must be right. Intuitively, the extra CD^2 term pushes AD^2 to equal the sum of squares from two perpendicular contributions meeting at C.

Step-by-Step Solution (outline):
In ΔABC with ∠B = 90°, AC^2 = AB^2 + BC^2.In ΔACD: by Cosine Rule, AD^2 = AC^2 + CD^2 − 2·AC·CD·cos(∠ACD).Given AD^2 = AB^2 + BC^2 + CD^2 = AC^2 + CD^2, hence −2·AC·CD·cos(∠ACD) = 0.Therefore cos(∠ACD) = 0 ⇒ ∠ACD = 90°.

Verification / Alternative check:
The equality reduces precisely to the Cosine Rule with the cosine term vanishing, which confirms a right angle at C on diagonal AC.

Why Other Options Are Wrong:
Any non-right angle would leave a nonzero cosine term, violating the given identity.

Common Pitfalls:
Applying Pythagoras directly to non-right triangles; here we must route through the Cosine Rule using AC as a bridge.

Final Answer:
90°