(A) Equation of a straight Line 

 An equation of straight line is given by y = mx + c.
• The variables x and y are linearly related
• The term c is known as y-intercept. It represents the y value where the line cuts the y-axis
• The term m is the gradient of the straight line and its value is constant

(B) Gradient of a Straight Line
If the points $A(x_1,y_1)$  and $B(x_2,y_2)$   lie on the straight line $y=mx+c$ , then gradient of ,the straight line
$$m=\frac{y_2-y_1}{x_2-x_1}or\frac{y_1-y_2}{x_1-x_2}$$

(C) Mid Point 



Mid Point AB is given by $$M=\left(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2}\right)$$

(D) Distance Between Two Points
Distance between point $A(x_1,y_1)$ and point is $B(x_2,y_2)$ given by $$\sqrt{{\left(x_1-x_2\right)}^2+{\left(y_1-y_2\right)}^2}$$

Linear Function (Straight Line)

Examples :
(a) y = 3x + 5
(b) 3y + 2x - 7 = 0

Non-Linear Function

Examples :
(a) $y=a{x}^3+b{x}^2$

(b) $y=ax+\frac{b}{x}$

(c) $y=a{b}^x$

(d) $y=\frac{x}{p+qx}$

A set of two variables are related non linearly can be converted to a linear equation.  The line of best fit can be written in the form

Y = mX + c

where
X and Y are in terms of x and/or y
m is the gradient,
c is the Y-intercept

Examples:
Reduce each of the following equations to the linear form. Hence, state the gradient and the Y-intercept of the linear equations in terms of  a and b.
(a)  $y=a{x}^3+b{x}^2$
(b)  $y=ax+\frac{b}{x}$
(c)  $y=ax-b{x}^2$
(d)  $xy=\frac{p}{x}+qx$
(e)  $y=a\sqrt{x}+\frac{b}{\sqrt{x}}$
(f)  $\frac{a}{y}=\frac{b}{x}+1$

[Note :
X and Y cannot have constant, but can have the variables (for example x and y)
 m and c can only have the constant(for example a and b), cannot have the variables x and y]

Solution:

(C) Mid Point 



Mid Point AB is given by $$M=\left(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2}\right)$$

2.2 The Line of Best Fit

Line of best fit has 2 characteristics:
(i) it passes through as many points as possible,
(ii) the number of points which are not on the line of best fit are equally distributed on the both sides of the line.