Atriangle has side lengths of 18 cm, 80 cm, and 81 cm. classify it as acute, obtuse, or right.
obtuse
right
acute

81^2 = 18 ^2 + 80^
6561 = 324 + 6400
6561 = 6724
It is not a right angle, since 18^2 + 80^2 does not equal 81^2.
So,
Using The law of cosines:
The largest angle C is opposite to the side c.
Therefore, 
c^2 = a^2 + b^2 -2ab cosC 
Here,
 c^2 < (a^2 + b^2)
 cosC must be positive which makes C an acute angle. 

the law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. in other words, a given compound will always contain the same elements in the same proportions by mass.the law of multiple proportions states when two elements combine with each other to form more than one compound, then the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.