Interpolation is the process of estimating the value of the function between two given data points. Linear Interpolation is one of the method for analysis. It is a straight line which passes between two given data points.
Consider two coordinates $(x_{a}, y_{a})$ and $(x_{b}, y_{b})$,a straight line passes between those points. For a value $y$,an equation is given as

        $\frac{(y - y_{a})}{(x - x_{a})} = \frac{(y_{b} - y_{a})}{(x_{b} - x_{a})}$

on solving this equation for $y$,we get
$y = y_{a} +$ $\frac{(y_{b} - y_{a})(x - x_{a})}{(x_{b} - x_{a})}$ this is a formula for linear interpolation for the interval $(x_{a} - x_{b})$.