IF a,b,c are real numbers and 3b^2-8ac

IF a,b,c are real numbers and 3b^2-8ac<0,then the equation ax^4+bx^3+cx^2+5x+7=0 cannot have all roots real. prove this.


1 Answers

mycroft holmes
272 Points
13 years ago

The wording of the question is a dead giveaway.


Suppose all the roots r1, r2, r3 and r4 are real, then we must have


so that


Now, from Vieta's formulas, . So, if all roots are real, we must necessarily have  


or .


Since we are given that  it implies that all roots cannot be real.

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