The population of a town in 2005 was 1800000 and in 2009 was 2100000. To find the increase in population, we can subtract the population in 2005 from the population in 2009.

- Calculation:
Population in 2009 - Population in 2005 = Increase in population

- Population in 2009 = 2100000
- Population in 2005 = 1800000

Therefore, the increase in population = 2100000 - 1800000 = 300000.

The population of the town increased by 300000 between 2005 and 2009.

Explanation of Congruence of Triangles:

Congruence of triangles refers to the condition when two triangles have exactly the same shape and size. In other words, if the corresponding sides and angles of two triangles are equal, they are said to be congruent.

- Corresponding Sides: The corresponding sides of two congruent triangles are equal in length. For example, if we have two triangles ABC and DEF and if side AB is equal to side DE, side BC is equal to side EF, and side AC is equal to side DF, then the triangles are congruent.

- Corresponding Angles: The corresponding angles of two congruent triangles are equal in measure. For example, if angle A is equal to angle D, angle B is equal to angle E, and angle C is equal to angle F, then the triangles are congruent.

- Congruence Criteria: There are various criteria to determine the congruence of triangles. Some of the commonly used criteria are:

1. Side-Side-Side (SSS) Criterion: If the three sides of one triangle are equal to the three sides of another triangle, then the triangles are congruent.

2. Side-Angle-Side (SAS) Criterion: If two sides and the included angle of one triangle are equal to the corresponding two sides and included angle of another triangle, then the triangles are congruent.

3. Angle-Side-Angle (ASA) Criterion: If two angles and the included side of one triangle are equal to the corresponding two angles and included side of another triangle, then the triangles are congruent.

4. Right-Angle-Hypotenuse (RHS) Criterion: If the hypotenuse and one of the other sides of a right-angled triangle are equal to the hypotenuse and one of the other sides of another right-angled triangle, then the triangles are congruent.

These criteria help us determine whether two given triangles are congruent or not. Congruence of triangles is an important concept in geometry and has applications in various fields such as engineering, architecture, and computer graphics.

300000