The Attack of the Mathematical Zombie: $(a+b)^2$

An occasional series highlighting common errors that refuse to die.

“It just… won’t stay dead!” he said, as the Mathematical Zombie moved closer.

“$(a+b)^2 = a^2 + b^2$”, it said. “Brains! $(a+b)^2 = a^2 + b^2$.”

“But… it doesn’t!” he said. “You have to multiply out the brackets!”

“$(a+b)^2 = a^2 + b^2$!”

“No! $(a+b)^2 = (a+b)(a+b) = a^2 + 2ab + b^2$”, he said, firmly.

“Oh,” said the Mathematical Zombie. “So you have to write both brackets down and multiply them out? $a^2$... $ab$... another $ab$... and $b^2$... makes $a^2 + 2ab + b^2$!”

While the Mathematical Zombie was distracted, he shot it in the head.

Colin

Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008. He lives with an espresso pot and nothing to prove.

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I teach in my home in Abbotsbury Road, Weymouth.

It's a 15-minute walk from Weymouth station, and it's on bus routes 3, 8 and X53. On-road parking is available nearby.

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