IThe numbers a,b and c are between 2 and 18 such that (i) their sum is 25 (ii) the numbers 2,a,b are in A.P (iii) the numbers b,c,18 are consecutive terms of a G.P.
f a,b,c are the roots of x3+qx2+rx+s=0 then the value of ′q′ is
A
−25
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B
−20
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C
−13
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D
0
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Solution
The correct option is D−25 (i) Sum=a+b+c=25 ........(1) (ii) The terms 2,a,b are in A.P ⇒2a=b+2 .........(2) (iii) The terms b,c,18 are in G.P ⇒c2=18b ....(3) From (1) and (2)a+b+c=25 ⇒a=25−b−c (on transposition from L.H.S to R.H.S) ⇒b+22=25−b−c (from (2)) ⇒b+2=50−2b−2c (Multiplying both sides by 2) ⇒b+2b=48−2c or 3b=48−2c From(3) substituting for b we get 2c+3c218=48 ⇒2c+c26=48 ⇒12c+c2−288=0 (Multiplying by 6) ⇒c2+12c−288=0 (Rearranging) ⇒(c−12)(c+24)=0 (on factorizing the above equation) Hence. c=12,−24c=12 is valid (∵a,b,c) lie between 2 and 18 ∴c=12⇒c2=18b (from (3)) Substituting c=12 in c2=18b we get b=14418=8 Substituting the values of b=8,c=12 in a+b+c=25 we get a=25−20=5 ∴a=5,b=8,c=12 From (1) we have a+b+c=25=−q ∴−q=25 or q=−25