What if there are three delta △ADB with D as the vertex? △ADC？ △CDB is an acute triangle,

then what ∠ ADB +∠ ADC +∠ BDC < 270 degrees,

But ABCD is on a plane,

These three angles all take d as the vertex, and the sum of the three angles is the angle of circumference, which is 360 degrees.

This contradicts < 270 degrees,

So delta delta △ADB? △ADC？ △CDB can't all be acute triangles.