If the expression y2+2xy+2x+my−3 can be resolved into two rational factors, then m must be
A
any possible real number
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B
any negative real number
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C
−2
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D
3
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Solution
The correct option is C−2 Treating it as a quadratic in y, we get y2+y(2x+m)+(2x−3)=0 D=(2x+m)2−4(2x−3) =4x2+4mx+m2−8x+12 =4x2+4x(m−2)+m2+12 =[4x2+4x(m−2)+(m−2)2]+(m2+12)−(m−2)2 =[4x2+4x(m−2)+(m−2)2]+[12+4m−4] =(2x+m−2)2+[12+4m−4] Hence, 4m=−8 ⇒m=−2.