Tip:
Highlight text to annotate it
X
In these problems, we're going to multiply or
divide signed numbers.
So let's take a look first at the rule.
If we multiply or divide 2 same signed
numbers, we get a positive.
If we multiply or divide two different signed numbers, we
get a negative.
So let's go ahead.
4 times 5.
Both of these are positive, same sign, and we
get positive 20.
And here we have a negative 4 times a positive 5.
So we have two different signs.
And two different signs means we get negative.
And then 4 times 5, 20.
And this is different than when we were adding and
subtracting signed numbers.
Because remember, if we were adding or subtracting--
let's say we were adding negative 4 plus 5.
Then we're not going to automatically say because
they're different signs we get a negative, but for
multiplication, we are going to do that.
So it's very different than the addition
and subtraction rules.
Negative 4 times negative 5, two same signs, positive.
And 4 times 5, 20.
So usually I find it helpful to just think about what's the
sign going to be first, and then do out the numbers.
And same for division.
Negative 10 divided by negative 2.
Well, first of all, negative, negative, two same signs, so
we get positive.
And 10 divided by 2 is 5.
So positive 5.
Here we have a positive divided by a negative, which
means we're going to get a negative.
15 divided by 3 is 5, so negative 5 is the answer.
And here we have fractions being divided.
Negative 3/2 divided by negative 9/4.
So the first thing I notice is that we have two same signs,
negative divided by negative.
So we're going to get a positive answer.
And then we want to do 3/2 times--
so change division to times--
the reciprocal, 4/9.
And I know my final answer is going to be positive because
it's a negative times a negative.
So multiplying these fractions now, let's see.
Let's cancel out some common factors.
So divide out a 2, which gives us a 1 and a 2.
And divide out a 3, which gives a 1 and 3.
And we end up with positive 1 times 2 over
1 times 3, or 2/3.