posted by Sandrine on .
How come when you find the square of the binomial (2x-5y)^2, the answer is 4x^2-20xy+25y^2 (according to my book)? I would think that the middle term, 2(2x-5y) would be 4x-10y, but my book says it is -20xy. But I thought you 2x and 5y were separate terms so you can't combine them like that by multiplying them together to form -20xy. Please explain why this is. Thanks!
FOIL: (x+y)^2= x^2+2xy+y^2