Triangle abc is an obtuse triangle with the obtuse angle at vertex b. angle a must be

Angle A must be acute.

Both angle A and C must be acute. The sum of the angles in a triangle is 180°.

An obtuse angle is more than 90°, so the sum of the remaining 2 angles has to be less than 90°.

Note that it is impossible to have:

It is possible to have an obtuse-angled isosceles triangle, but the vertex angle must be obtuse and the equal base angles will be acute.

Option A on Edge is correct, less than 90 degrees

Step-by-step explanation:

got it right

12

step-by-step explanation:

a triangle's   full angle is 180 degrees so 180 - 156 = 24. 24/2=12