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Q.

Find all real numbers m such that x2+my24my+6y6x+2m+80 If K is the largest value of m then find the value of 2K is______.

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answer is 9.

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Detailed Solution

This looks rather complicated. one nice thing about this inequality is that the highest degree terms of x and y are both of degree 2, which allows us to use properties of quadratic functions. Interpret the inequality as a quadratic function in x. Rearranging terms, we have

x26x+my24my+6y+2m+80.
Since the left hand side is nonnegative, and the leading coefficient is positive, it follows that the discriminant, Δ, of this quadratic in x must be nonpositive. In particular, for all y, we must have
Δ=624(my24my+6y+2m+8)0.

This is still a fairly complicated condition. But again, the highest degree term of y is of degree 2, so we can follow the same strategy and interpret the discriminant as a quadratic in y. Since the quadratic must be nonpositive for all y, the leading term must be negative, which implies m>0.. As a quadratic in y, we can rearrange terms to obtain
Δ=4(my2+2y(2m3)2m+1)0.
We can repeat the process, and take the discriminant of our nonpositive quadratic in y. The new discriminant function, Δ^', should be nonpositive. Hence
Δ=4((2m3)2m(2m1))0

This is equivalent to a quadratic inequality in m, that is
2m211m+9=2(m1)(m92)0

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