1. (20 points) Given that a, b, and c are real numbers, ac<0, and 2a+3b+5c=0. Prove: The quadratic equation ax2+bx+c=0 has a root greater than 43 and less than 1.
Solution
1. From the condition, we have 53b+c=−52a. Let y=ax2+bx+c.
When x=53, we have y1=53a+53b+c=53−10a.
When x=1, we have y2=a+b+c=a+b+c−31(2a+3b+5c)=3a[(3−2)−ac(5−3)].
Since 3−10<0,5−3>0, and −ac>0, then y1y2=533−10[(3−2)a2−ac(5−3)a2]>43, hence the equation has a root greater than 43 and less than 1.
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