Christmas Streak 24/88: Hard To Be Always Positive

Algebra Level 4

a x 2 + b x + c ax^2+bx+c is positive for all reals x . x.

Given that a , b , c a,~b,~c are integer constants that satisfy a + c = 5 , a+c=5, find the sum of all the possible values for a 2 + b 2 + c 2 . a^2+b^2+c^2.


This problem is a part of <Christmas Streak 2017> series .

294 160 147 370 185 129 25 320

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1 solution

Boi (보이)
Oct 21, 2017

We don't know whether a x 2 + b x + c ax^2+bx+c is quadratic. Therefore, we must split the cases.


i ) a 0 i)~a\neq 0

Then the discriminant D = b 2 4 a c < 0. D=b^2-4ac<0. Therefore b 2 < 4 a c . b^2<4ac.

Since a + c = 5 , a+c=5, ( a , c ) = ( 1 , 4 ) , ( 2 , 3 ) , ( 3 , 2 ) , ( 4 , 1 ) . (a,~c)=(1,~4),~(2,~3),~(3,~2),~(4,~1). However, the last two cases would yield the same values for a 2 + b 2 + c 2 a^2+b^2+c^2 with the first two cases.

So we're going to look at ( a , c ) = ( 1 , 4 ) , ( 2 , 3 ) . (a,~c)=(1,~4),~(2,~3).

If a = 1 , c = 4 , a=1,~c=4, then b 2 < 16. b^2<16. Possible values for b 2 b^2 are 0 , 1 , 4 , 9. 0,~1,~4,~9.

a 2 + b 2 + c 2 = 17 , 18 , 21 , 26. \therefore~a^2+b^2+c^2=17,~18,~21,~26.

If a = 2 , c = 3 , a=2,~c=3, then b 2 < 24. b^2<24. Possible values for b 2 b^2 are 0 , 1 , 4 , 9 , 16. 0,~1,~4,~9,~16.

a 2 + b 2 + c 2 = 13 , 14 , 17 , 22 , 29. \therefore~a^2+b^2+c^2=13,~14,~17,~22,~29.

From above, the possible values are 13 , 14 , 17 , 18 , 21 , 22 , 26 , 29. 13,~14,~17,~18,~21,~22,~26,~29.


( i i ) a = 0 (ii)~a=0

Then b x + c > 0 , bx+c>0, but if b 0 , b\neq 0, this doesn't make sense, because a linear graph (except the constant one) always passes through the x x -axis.

So b = 0 , b=0, and since a + c = 5 , c = 5. a+c=5,~c=5.

a 2 + b 2 + c 2 = 25. \therefore~a^2+b^2+c^2=25.


From above, the sum of all the possible values for a 2 + b 2 + c 2 a^2+b^2+c^2 is 13 + 14 + 17 + 18 + 21 + 22 + 25 + 26 + 29 = 185 . 13+14+17+18+21+22+25+26+29=\boxed{185}.

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