perimeter ≈ 18.9 units ( to 1 dec. place )
Calculate the lengths of the 3 sides using the distance formula
d = √( (x₂ - x₁ )² + (y₂ - y₁ )²
with (x₁, y₁ ) = A(1, 1) and (x₂, y₂ ) = B(9, 2 )
d = √( (9 - 1 )² + (2 - 1 )² ) = √(64 + 1 ) = √65
repeat with (x₁, y₁ ) = B(9, 2) and (x₂, y₂ ) = C(4,5 )
d = √( (4 - 9 )² + (5 - 2 )² ) = √(25 + 9 ) = √34
let (x₁, y₁ ) = A(1, 1 ) and (x₂, y₂ ) = C(4, 5 )
d = √( (4 - 1 )² + (5 - 1 )² ) = √(9 + 16 = √25 = 5
perimeter = √65 + √34 + 5 ≈ 18.9 units
