The total sales of a firm selling products A and B are given by the
relationships V=a+bx+cy . The data for 3 months period is given below:
Months  Volume of sales(V) (Rs.)   x     y
April       15,000                                3   4
May       18,000                                5   2
June      20,000                                6   3
Using determinant method, determine the sales in the next month when it sells 8
units of x and 5 units of y . If a, b, c are all positive and the pth, qth and rth terms
of a Geometric Progression (GP), then prove that
[loga  p   1
 logb    q  1
 logc     r   1 ]=0