Answer:
We can describe FOIL using the distributive property of multiplication. For real numbers x, y, and z the distributive property of multiplication states that
[tex](x + y)z = xz + yz[/tex]
Now let [tex]z = v + w[/tex], where v and w are real numbers.
Then [tex](x + y)(v + w) = x(v + w) + y(v + w)[/tex]
Here, we can again use the distributive property of multiplication to get
[tex]x(v + w) + y(v + w) = xv + xw + yv + yw[/tex]
Hence, it follows that
[tex](x + y)(v + w) = xv + xw + yv + yw[/tex]
This is the method of FOIL (first: xv, outside: xw, inside: yv, last: yw)
For example, [tex](1 + 2)(3 + 4) = 21 \\\text{and} \\1(3) + 1(4) + 2(3) + 2(4) = 3 + 4 + 6 + 8 = 21[/tex]