# Solution to the [[Two Triangles and a Quarter Circle]] Puzzle +-- {.image} [[TwoTrianglesandaQuarterCircle.jpeg:pic]] > The green triangles are congruent. What’s the angle? =-- ## Solution by [[Angle in a Semi-Circle]] +-- {.image} [[TwoTrianglesandaQuarterCircleReflected.jpeg:pic]] =-- In the above diagram, the original two triangles are reflected in the horizontal edge of the quarter-circle, and the quarter-circle is completed to a full circle. As the green triangles are [[congruent]], $D C A$ is a straight line. Therefore, angle $A \hat{D} B$ is the [[angle in a semi-circle]] so is $90^\circ$.