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Question

The entire graph of the equation y=x2+kxx+9 is strictly above the xaxis if and only if :

A
k<7
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B
5<k<7
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C
k>5
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D
none of these
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Solution

The correct option is B 5<k<7
y=x2+kxx+9
a>0
if b24ac<0 & a>0
then the graph will always be above x-axis
a=1a>0.
(K1)24×9<0
(K1)262<0
(K16)(K1+6)<0
(K7)(K+5)<0
K>5,K<7
5<K<7
Option B.

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