If one angle of a triangle is equal to the sum of the other two angles, then the triangle must be

Let the three angles of the triangle be $A$, $B$, and $C$.

Given: $A = B + C$

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In any triangle, the sum of all angles is $180°$:

$A + B + C = 180°$

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$(B + C) + B + C = 180°$

$2B + 2C = 180°$

$2(B + C) = 180°$

$B + C = 90°$

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Since $A = B + C$:

$A = 90°$

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A triangle with one angle equal to $90°$ is a right triangle.

Therefore, the triangle must be a RIGHT TRIANGLE.