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Algebra Difficulty 2.8 Junior Find the answer

There are two values of kk for which the equation x2+2kx+7k10=0x^{2}+2kx+7k-10=0 has two equal real roots (that is, has exactly one solution for xx). What is the sum of these values of kk?

A number or a short expression. Spacing and $ signs are ignored.

Solution

The equation x2+2kx+7k10=0x^{2}+2kx+7k-10=0 has two equal real roots precisely when the discriminant of this quadratic equation equals 0. The discriminant, Δ\Delta, equals Δ=(2k)24(1)(7k10)=4k228k+40\Delta=(2k)^{2}-4(1)(7k-10)=4k^{2}-28k+40. For the discriminant to equal 0, we have 4k228k+40=04k^{2}-28k+40=0 or k27k+10=0k^{2}-7k+10=0 or (k2)(k5)=0(k-2)(k-5)=0. Thus, k=2k=2 or k=5k=5. We check that each of these values gives an equation with the desired property. When k=2k=2, the equation is x2+4x+4=0x^{2}+4x+4=0 which is equivalent to (x+2)2=0(x+2)^{2}=0 and so only has one solution for xx. When k=5k=5, the equation is x2+10x+25=0x^{2}+10x+25=0 which is equivalent to (x+5)2=0(x+5)^{2}=0 and so only has one solution for xx. The sum of these values of kk is 2+5=72+5=7.

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