Question

a triangle whose angle measure are 51 , 90 and 39 is a triangle
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Answer 1

51+90+39

180

The measure of interior angle of triangle is 180

it is a triangle

Answer 2

If there is at least one right angle in the triangle then you have a right angled triangle. since you have 90 degrees you have a right triangle

To verify if it is a triangle then you can add up all the angles. if the angles sum upto 180° you have a triangle

51°+39°+90°= 180°

hence u have a triangle!

Hope this was helpful :)