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## Substitution and evaluating expressions

## Video transcript

Now, let's think
about expressions with more than one variable. So say I had the
expression a plus-- I'll do a really
simple one, a plus b. And I want to evaluate this
expression when a is equal to 7 and b is equal to 2. And I encourage you to pause
this and try this on your own. Well, wherever we see
the a, we would just replace it with the 7. And wherever we see the b,
we'd replace it with the 2. So when a equals
7 and b equals 2, this expression
will be 7 plus 2, which, of course, is equal to 9. So this expression
would be equal to 9 in this circumstance. Let's do a slightly
more complicated one. Let's say we have the expression
x times y minus y plus x. Actually, let's make it plus 3x. Or another way of saying
it plus 3 times x. So let's evaluate this when x is
equal to 3 and y is equal to 2. And once again, I encourage
you to pause this video and try this on your own. Well, everywhere we see an
x, let's replace it with a 3. Every place we see a y,
let's replace it with a 2. So this is going to
be equal to 3 times y. And y is 2 in this case. 3 times 2 minus 2
plus this 3 times x. But x is also now equal to 3. So what is this
going to be equal to? Well, this is going to be
equal to 3 times 2 is 6. This 3 times 3 is 9. So it simplifies
to 6 minus 2, which is 4, plus 9, which
is equal to 13. So in this case,
it is equal to 13.