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Both at Once

Part of the Functions & Transformations section of Coddy's Algebra 2 journey. Lesson 33 of 55.

The two kinds of move combine. Take y=f(x+1)−2y = f\left(x+1\right)-2 with f(x)=x2f\left(x\right) = {x}^{2}. Inside, x+1x+1 moves the graph 1 left. Outside, the minus 2 moves it 2 down. So the vertex goes from (0, 0) to (-1, -2).

The pattern is worth naming: for f(x)=x2f\left(x\right) = {x}^{2}, y=f(x−h)+ky = f\left(x-h\right)+k has its vertex at (h, k). Read h from inside the brackets, where the sign looks flipped (x+1x+1 is x−(−1)x-\left(-1\right), so h is -1), and read k from outside, exactly as written. With f(x)=x2f\left(x\right) = {x}^{2} this is y=(x−h)2+ky = {\left(x-h\right)}^{2}+k, called the vertex form of a parabola.

The order of the two moves does not matter: slide then lift, or lift then slide, you land in the same place. After the vertex, the shape does the rest. One step right of it the curve is 1 higher, so (0, -1) is on this graph too.

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Challenge

Draw the graph of y=f(x−1)+2y = f\left(x-1\right)+2 for f(x)=x2f\left(x\right) = {x}^{2}. The faint curve is f itself. Drag the vertex, then the second point.

Try it yourself

y=f(x−1)+2y = f\left(x-1\right)+2

Draw the parabola

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