Linear meets Quadratic

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79 80 81 78

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1 solution

Tom Engelsman
Jan 18, 2021

If the parabola and the line above intersect at two distinct points, then we have the quadratic equation:

x 2 + ( a 19 ) x + ( b + 1 ) = 0 x = ( 19 a ) ± ( a 19 ) 2 4 ( 1 ) ( b + 1 ) 2 = ( 19 a ) ± a 2 38 a + 361 4 b 4 2 = ( 19 a ) ± a 2 38 a 4 b + 357 2 x^2 + (a-19)x + (b+1) = 0 \Rightarrow x = \frac{(19-a) \pm \sqrt{(a-19)^2 - 4(1)(b+1)}}{2} = \frac{(19-a) \pm \sqrt{a^2 - 38a + 361 - 4b -4}}{2} = \frac{(19-a) \pm \sqrt{a^2 - 38a -4b + 357}}{2} .

If x = 9 3 x = 9-\sqrt{3} is one of the x x- coordinates of intersection, then we require:

19 a 2 = 9 a = 1 \frac{19-a}{2} = 9 \Rightarrow \boxed{a = 1} ;

a 2 38 a 4 b + 357 = 12 b = 77 a^2 - 38a - 4b + 357 = 12 \Rightarrow \boxed{b = 77} ;

in order to attain x = 9 ± 3 x = 9 \pm \sqrt{3} as the two distinct x x- coordinates of intersection. Hence, a + b = 78 . \boxed{a+b=78}.

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