y = 2 x^2 – 13 x – 45

2y = 4 x^2 – 26 x – 90 = (2x)^2 – 13 (2x) – 90

2y = ((2x) – 18)((2x) + 5)

y = (x – 18)(2x + 5)

Where I have (2x) in the above the video just writes x… It would be clearer (and less dodgy) if a temporary variable w=2x was introduced. I’m not a big fan of this method.

By the way, the underlying trick seems to be similar to James Tanton’s prefered way of completing the square.

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