A) What is the length of MN? b) ABC is a scalene triangle. If AB=6 cm and BC=4 cm. c) In RST, RS=ST=TR=9 cm, find the perimeter.

Here are the names of the triangles:

a) The triangle has angles $90^\circ$, $60^\circ$, and $30^\circ$. Since it has one angle equal to $90^\circ$, it is a right-angled triangle.

b) The triangle has angles $100^\circ$, $45^\circ$, and $35^\circ$. Since it has one angle greater than $90^\circ$ ($100^\circ$), it is an obtuse-angled triangle.

c) The triangle has angles $80^\circ$, $60^\circ$, and $40^\circ$. Since all its angles are less than $90^\circ$, it is an acute-angled triangle.