is positive for all reals
Given that are integer constants that satisfy find the sum of all the possible values for
This problem is a part of <Christmas Streak 2017> series .
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We don't know whether a x 2 + b x + c is quadratic. Therefore, we must split the cases.
i ) a = 0
Then the discriminant D = b 2 − 4 a c < 0 . Therefore b 2 < 4 a c .
Since a + c = 5 , ( 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 with the first two cases.
So we're going to look at ( a , c ) = ( 1 , 4 ) , ( 2 , 3 ) .
If a = 1 , c = 4 , then b 2 < 1 6 . Possible values for b 2 are 0 , 1 , 4 , 9 .
∴ a 2 + b 2 + c 2 = 1 7 , 1 8 , 2 1 , 2 6 .
If a = 2 , c = 3 , then b 2 < 2 4 . Possible values for b 2 are 0 , 1 , 4 , 9 , 1 6 .
∴ a 2 + b 2 + c 2 = 1 3 , 1 4 , 1 7 , 2 2 , 2 9 .
From above, the possible values are 1 3 , 1 4 , 1 7 , 1 8 , 2 1 , 2 2 , 2 6 , 2 9 .
( i i ) a = 0
Then b x + c > 0 , but if b = 0 , this doesn't make sense, because a linear graph (except the constant one) always passes through the x -axis.
So b = 0 , and since a + c = 5 , c = 5 .
∴ a 2 + b 2 + c 2 = 2 5 .
From above, the sum of all the possible values for a 2 + b 2 + c 2 is 1 3 + 1 4 + 1 7 + 1 8 + 2 1 + 2 2 + 2 5 + 2 6 + 2 9 = 1 8 5 .