There are two values of k for which the equation x2+2kx+7k−10=0 has two equal real roots (that is, has exactly one solution for x). What is the sum of these values of k?
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Solution
The equation x2+2kx+7k−10=0 has two equal real roots precisely when the discriminant of this quadratic equation equals 0. The discriminant, Δ, equals Δ=(2k)2−4(1)(7k−10)=4k2−28k+40. For the discriminant to equal 0, we have 4k2−28k+40=0 or k2−7k+10=0 or (k−2)(k−5)=0. Thus, k=2 or k=5. We check that each of these values gives an equation with the desired property. When k=2, the equation is x2+4x+4=0 which is equivalent to (x+2)2=0 and so only has one solution for x. When k=5, the equation is x2+10x+25=0 which is equivalent to (x+5)2=0 and so only has one solution for x. The sum of these values of k is 2+5=7.
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