Answer:
21/4 = 5.25
Step-by-step explanation:
You want the sum of the x-values that are solutions to ...
21x -4x² = 5
4x² -21x +5 = 0 . . . . . subtract the left side, rearrange to standard form
x² -(21/4)x +5/4 = 0 . . . divide by 4
In this form the sum of the x-values that solve this equation is the opposite of the x-coefficient. That sum will be 21/4 = 5.25. (The attached graph confirms this result.)
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Consider a quadratic with roots p and q. It factors as ...
(x -p)(x -q) = 0 = x² -(p+q)x +pq
That is, the x-coefficient is the opposite of the sum of the roots p and q.
