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Is the number of corners in the mathematical graphics of the second grade of primary school only an inner corner? Is it necessary to master the corner where the other two corners are combined?
There is a way to establish a vertex, look at how many angles there are on a vertex, count the next one, and finally sum it up.

Such as trapezoidal ABCD, AC is diagonal, and there are three corners with a vertex: ∠ DAC, ∠ Cab, ∠ DAB;

There is an angle with b as the vertex: ∠ b;

There are three angles with C as the vertex: ∠ BCA, ∠ ACD, ∠ BCD;

The vertex of an angle is d: ∠ d.

A * * * has 3+ 1+3+ 1=8.