If the roots of the equation bx^{2}+ cx + a = 0 be imaginary, then for all real values of x, the expression 3b^{2}x^{2} + 6bcx + 2c^{2} is
greater than 4ab
less than 4ab
greater than -4ab
less than 4ab
C.
greater than -4ab
As, bx^{2} + cx + a = 0 has imaginary roots
So, c^{2}< 4ab
Now, 3b^{2}x^{2} + 6bcx + 2c^{2}
= 3(bx + c)^{2}– c^{2}≥ – c^{2}≥ – 4ab
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