PSAT #1, Section 3, #13

General rule: when a question gives you a function and a point on that function, plug the point into the function. In this case, that means plugging (-1, 1) into y=ax^2+bx+c:

    \begin{align*}y&=ax^2+bx+c\\1&=a(-1)^2+b(-1)+c\\1&=a-b+c\end{align*}

And you’re already done! For the parabola to pass through that point, a-b+c must equal 1.

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