Not a fan of "cross multiply". Just another example of "lay" arithmetic. Learn to multiply. Learn how to find a common denominator. There is just no need to memorize a single-use method in algebra. Stick to basic principles.
My views. I welcome others'.
Not a fan of "cross multiply". Just another example of "lay" arithmetic. Learn to multiply. Learn how to find a common denominator. There is just no need to memorize a single-use method in algebra. Stick to basic principles.
My views. I welcome others'.
I do, respectfully, disagree.
[MATH]a + b = c \implies (a + b) - b = c - b \implies a + (b - b) = c - b \implies a + 0 = c - b \implies a = b - c.[/MATH]
That is using the basics. But I bet we all "flip signs" and in a lay way go
[MATH]a + b = c \implies a = b - c.[/MATH]
There are many short-cuts that save time in algebra like the quadratic formula. So long as students
clearly understand the reasoning behind a short-cut, I see no reason to discourage its use.
Cross multiplication is just another short-cut, though one easily misapplied.
[MATH]\dfrac{a}{b} = \dfrac{c}{d} \implies ad = bc[/MATH] just skips numerous steps in the non-lay way that is formally justified with
[MATH]\dfrac{a}{b} = \dfrac{c}{d} \implies \dfrac{bd}{1} * \dfrac{a}{b} = \dfrac{bd}{1} * \dfrac{c}{d} \implies \dfrac{ad}{1} * \dfrac{b}{b} = \dfrac{bc}{1} * \dfrac{d}{d} \implies[/MATH]
[MATH](ad) * 1 = (bc) * 1 \implies ad = bc.[/MATH]
The only difference between "flipping signs" and "cross multiplication" is that the latter is a reliable way to clear fractions only in a special case. Students make mistakes when clearing fractions because they forget that cross multiplication does not work generally. This is a reason to make sure they understand why cross-multiplication works and when it does not apply. If they understand clearing fractions generally, then they are ready to understand and use cross multiplication.