The Brainliest Answer!
As every one know the sum of all angles of a triangle is 180 degrees and suppose we take one angle 90 degrees and second 80 degrees so to get the third we would have to add 90 + 80 = 170 and then 180 - 170 
so the answer is 10 degrees hope it is clear to you
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You can find the angle by angle sum property of triangle ,that is,  sum of all angles os a triangle is 180 . 
1. in an equilateral triangle all angles measure 60
2. in isosceles triangle angle opposite to equal sides are equal
3. in scalen triangle all angles are unequal

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