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### Course: Algebra 2 > Unit 12

Lesson 3: Manipulating formulas# Manipulating formulas: area

Sal rewrites the formula for the area of a triangle so it is solved for height. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- At about1:30, why does multiplying both sides of the equation by 2 not make it

2A = 2B2H?

I thought when you multiply both sides of the equation by 2 that every term is multiplied by 2.(14 votes)- This is actually a common question. You are partially right that when you multiply both sides of an equation by a number, you are applying each term by the number. But a term is a set of numbers and variables multiplied together.

One way to look at this is to think of a real example using numbers. Let's say our base is 3 and our height is 7. Our equation becomes:

A=1/2(3)*(7)

or A = 1/2 (21) = 21/2

If we multiply both sides by 2 we get:

2A =21 = (3)(7), not 2A = (2*3)*(2*7)(20 votes)

- I can't find any practice problems for this Skill... Any help??(5 votes)
- Look on this video to the left side. You'll see a playlist. At the bottom there is a skill to practice. Try this: //https://www.khanacademy.org/math/algebra/solving-linear-equations-and-inequalities/solving_for_variable/e/solving_for_a_variable(3 votes)

- What about volume,length,and width?(4 votes)
- triangles dont have a definite width, so there's no such thing(3 votes)

- Are there practice questions for this video?(3 votes)
**Solving equations in terms of a variable**is a skill on the knowledge map in the challenge for "Creating and solving linear equations.''(4 votes)

- I Think You Guys Have Great Description Of Topics And Are Very Helpful But I Was Wondering How You Know How To Get Rid Of The Numbers Like 1/2 at about1:08?

Thanks,

Courtnee(3 votes)- Okay Sal was solving for the height h of a triangle.

The formula for finding the area of a triangle is

A=(1/2)bh

And your wondering how can we solve for h if we have 1/2 in the other side, right?`The key realization in solving for any variable is to realize that to solve for it we basically just try to isolate it to one side of the equation by doing the same things on both sides of the equation`

Let's solve for height h of the triangle now.

A=(1/2)bh →This is our equation.

2A=bh →I multiplied both sides by 2 to get rid of the fraction 1/2

2A/b=h →I divided both sides by b to completely isolate h and there's our answer!(2 votes)

- I have no idea what this is, i am SO confused(4 votes)
- This is called solving literal equations. We can move variables from one side to another by opposites. So if you know A=lw (area of a rectangle is length times width), we can solve for either l or w by dividing, so l = A/w and w=A/l. We can do the same for any equation including A=1/2 bh. Multiply by 2 then divide by b (or h) to get b=2A/h and h=2A/b.(0 votes)

- Not sure how to solve the word problem: Justin has 7.50$ more than Eva, and Emma has 12$ less than Justin does. How much money does each person have if they have a total of 63$?(3 votes)
- Quincy,

There are three equations and three unknowns hidden in the words.

Use J for the amount for Justin

Use E for the amount for Eva

Use M for the amount for Emma

"Justin as $7.50 more than Eva" can be written as

J = E + 7.50

Emma has 12$ less than Justin

M = J - 12

They have a total of 63$

J + E + M = 63

So your three equations are

J = E + 7.50

M = J - 12

J + E + M = 63

I hope that helps make it click for you.(1 vote)

- how would we solve for the base?(3 votes)
- You solve for the base in the same way you solve for the height.(1 vote)

- at1:30why does 1/2 B become 1B, but H does not become 2H? Math, such a confusing subject xD.

Thanks for the help,

BlackOpsBanana(2 votes)- This same question has been asked multiple times. Read the questions and answers under this video and see if they help. The 2 highest voted responses answer the question well.(3 votes)

- Why not subtract 1/2b from both sides.?

A=1/2 bh

A-1/2b=h(2 votes)

## Video transcript

The formula for the
area of a triangle is A is equal to 1/2 b times
h, where A is equal to area, b is equal to
length of the base, and h is equal to the
length of the height. So area is equal to
1/2 times the length of the base times the
length of the height. Solve this formula
for the height. So just to visualize
this a little bit, let me draw a triangle here. Let me draw a triangle just
so we know what b and h are. b would be the
length of the base. So this distance
right over here is b. And then this distance
right here is our height. That is the height
of the triangle-- let me do that at a lower
case h because that's how we wrote it in the formula. Now, they want us to solve
this formula for the height. So the formula is area is
equal to 1/2 base times height. And we want to solve for h. We essentially want
to isolate the h on one side of the equation. It's already on the
right-hand side. So let's get rid of everything
else on the right-hand side. So we can do it-- well, I'll
do it one step at a time. We could kind of skip
steps if we wanted to. But let's see if we can
get rid of this 1/2. So the best way to get
rid of a 1/2 that's being multiplied by
h is if we multiply both sides of the equation
by its reciprocal. If we multiply both sides of
the equation by 2/1 or by 2. So let's do that. So let's multiply--
remember anything you do to one side
of the equation, you also have to do to the
other side of the equation. Now, what did this do? Well, the whole point
behind multiplying by 2 is 2 times 1/2 is 1. So on the right-hand
side of the equation, we're just going to have a bh. And on the left-hand side of
the equation, we have a 2A. And we're almost there, we
have a b multiplying by an h. If we want to just
isolate the h, we could divide both sides
of this equation by b. We're just dividing both sides. You can almost view b as
the coefficient on the h. We're just dividing
both sides by b. And then what do we get? Well, the right-hand
side, the b's cancel out. On the left-hand side, we're
just left with 2A over b. So we get h-- and I'm just
swapping the sides here. h is equal to 2A over b. And we're done. We have solved this
formula for the height. And I guess this
could be useful. If someone just gave
you a bunch of areas and a bunch of base
lengths, and they said keep giving me the
height for those values, or for those triangles.