this triangle with side length 3,4,5 is a right triangle because 3,4 and 5 is pythagora's triplet
so than you drawn this triangle and drawn thei height from right angle on hypotenuse so writing
sin30 = height/4
1/2 = h/4
4=2h
h = 2
knowing the property that the radius of a circle circumscribed to a triangle is equal 2/3 part of the length of the height of this triangle
in this case r = (2/3)*2 = 4/3
formula of circumference of a circle = 2pi*r
so in this case
C = 2*3,14*(4/3) = 6,28*4/3 =
= 25,12/3 = 8,373
hope this iwll help you
