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Grade 12th passAlgebra

Please solve the question in the attachment.... .... ....... .. . ..

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Profile image of  Pawan joshi
7 Years agoGrade 12th pass
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1 Answer

Profile image of Aditya Gupta
7 Years ago
the roots cannot be rational (option 3).
the sol is EXTREEEEEMELY HUGE.
first, use GP sum formula to obtain polynomial as (x^k+1)/(x+1). for this to be a polynomial, k needs to be odd. so, p and q are odd.
now consider the eqn 3x^2+px+5q=0. obviously all the coeffs are odd. now we shall prove that ax^2+bx+c=0 can never have rational roots as long as all of a, b and c are odd integers.
assume to the contrary that roots can be rational
now consider the determinant b^2 – 4ac. if roots are rational, this needs to be a square number, say m^2. but m will be odd (since odd – even= odd).
now rewrite it as b^2 – m^2= 4ac
since b is odd and we have just proved that m is odd as well, hence b^2= 8j+1 and m^2= 8i+1 (since any odd number square is always of the form 8k+1, however this would require another proof).
hence b^2 – m^2= 8*t=4ac
or ac= 2t
but since a and c are both odd, their product has to be an odd number, which is a contradiction since it is 2t in the above eqn. hence our initial assumption was wrong and therefore the roots can be anything BUT rational.