There are 6 books on mathematics, 4 books on physics annd 5 books on chemistry in a book shop. The number of ways can a student purchase either a book on mathematics or a book on chemistry, is

Options

A. 2
B. 4
C. 6
D. 8

Answer

B

Explanation

It is given that a,b,c are in G.P. and the AM of a,b,c is b+2.
`:." "b^(2)=acand(a+b+c)/(3)=b+2`
`rArr" "b^(2)=acanda+c=2b+6`
`rArr" "c=(b^(2))/(a)anda-2b+c=6`
`rArr" "a-2b+(b^(2))/(a)=6`
`rArr" "((b)/(a))^(2)-2((b)/(a))+1=(6)/(a)`
`rArr" "((b)/(a)-1)^(2)=(6)/(a)`
`rArr" "(6)/(a)` is a perfect square of integer `[because(b)/(a)" is an integer"]`
`rArr" "a=6`
`:." "(a^(2)+a-14)/(a+1)=(36+6-14)/(6+1)`