A number is divisible by 9 if the sum of its digits is divisible by 9.

Factors of a number are the integers that can divide that number without leaving a remainder, while multiples are the result of multiplying that number by an integer. Hence, factors are always less than or equal to multiples.

In a harmonic progression, the terms can indeed be negative, as the definition does not restrict the terms to positive values.

In a harmonic progression, the reciprocals of the terms indeed form an arithmetic progression.

HCF and LCM can be equal when both numbers are the same.

Modular arithmetic is primarily applicable to integers, making the first statement true.

Prime numbers are defined as having exactly two distinct positive divisors: 1 and the number itself.

Compound interest will always be greater than simple interest for the same principal, rate, and time, except for the case of zero interest.

When two lines intersect, the angles opposite each other (vertical angles) are always equal. Therefore, the correct statement is that vertical angles are always equal.

The base-8 number system, also known as octal, uses digits from 0 to 7. Therefore, the correct statement is that it uses digits 0-7.

All quadrilaterals are indeed polygons, as they are defined as shapes with four sides, which fits the definition of a polygon.

The composition of two functions can result in a function or not, depending on the nature of the original functions.

The presence of outliers is evident in the graph, which affects the overall trend of the data.

The property of modular arithmetic states that (a + b) mod n = [(a mod n) + (b mod n)] mod n.

The property of exponents states that a^(m+n) = a^m * a^n.

36 is indeed a perfect square as it can be expressed as 6 x 6.

The function f(x) = x^2 is a quadratic function because it can be expressed in the form ax^2 + bx + c.

The graph of a linear equation is always a straight line.

The graph of a quadratic function is a parabola that can open upwards (if a > 0) or downwards (if a < 0).

Setting y = 0 gives x = 4, indicating that the graph intersects the x-axis at (4, 0).

Rearranging the equation to y = -5/2x + 5 shows that the slope is negative.

The equation y = 2x + 1 has a positive slope of 2 and intersects the y-axis at 1.

In the equation y = mx + b, m represents the slope, which indicates how steep the line is.

The correct statement is that HCF * LCM = Product of the numbers.

The intersection of two constraint-based sets may contain elements that satisfy the constraints of both sets, and it will be empty if there are no common elements.

The intersection of two hybrid sets contains only the elements that are common to both sets.

The graph of a function and its inverse are symmetric about the line y = x, which is a key property of inverse functions.

The distribution of data points shows a right skew, indicating more values on the lower end.

Zero is neither positive nor negative in any number system.

'100' in base 3 is calculated as 1*3^2 + 0*3^1 + 0*3^0 = 9.