Question

define a triangle and classify them on the basis of sides and angle

Answer 1

A triangle is a 3-sided polygon made up of only line segments.
On the basis of sides
i.Equilateral triangle
ii.Scalene triangle
iii. Issoscles triangle

On the basis of angle
i.Right angled triangle
ii.Obtuse angled triangle
iii.Acute angled triangle

Hope it helps