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Question

If x2+5x+7=0 and ax2+bx+c=0 have a common root and a,b,c N, then the minimum value of a+b+c is .

A
1
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B
5
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C
7
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D
13
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Solution

The correct option is D 13
The roots of equation x2+5x+7=0 are non-real. And we know that if a quadratic equation with real coefficients have non real roots, then they occur in conjugate pair.
We are given that the both equations have a common root and a,b,c are natural numbers. It means the roots of the equation ax2 + bx + c = 0have complex roots in conjugate pair. Thus, the both equations have both common roots.
So, a1= b 5=c7=k, kNa=k, b=5k, c=7ka+b+c=13kHence, the minimum value of a+b+c is 13

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