Let 
be the length of the leg with one tick mark and 
the length of the leg with two tick marks.
In the upper triangle, the law of cosines says
In the lower triangle, it says
Subtract the second equation from the first to eliminate 
:
and are lengths so they must both be positive. 10 is also positive, so in order to preserve the sign on both sides of this equation, we must have
Now we have to be a bit careful. If 
is an acute angle, then as gets larger, the value of 
gets smaller. So if we have two angles 
and 
, with 
, then we would have 
.
This means in our inequality, taking the inverse cosine of both sides would reverse the inequality:
We know that 
is an angle in a triangle, so it must be some positive measure:
So we must have
Ángulo exterior, la medida de un ángulo exterior de un triángulo es igual a la suma de las medidas de los dos ángulos interiores no adyacentes.
espero que haya ayudado, por favor, marcame el cerebro.
Q: If the 3 inner angles of a triangle are congruent, what is the measure of each one?
A: Exterior Angle The measure of an exterior angle of a triangle is equal to the sum of the mesures of the two non-adjacent interior angles. Hope it helped please mark me brainliest/
DF congruent to AC and EF is common side
