Triangle Area 21: Calculate the Height X with Base 7

Triangle Area Formula with Height Calculation

The area of the triangle below is equal to 21.

Calculate X.

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:05 Let's find the height, X.
00:08 We'll use the triangle area formula.
00:11 It's Base B C times height A E, divided by 2.
00:16 Now, plug in the numbers and solve for height, X.
00:26 Double it to get rid of the fraction.
00:32 Keep solving until you isolate X.
00:42 And that's how we find the solution!

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

The area of the triangle below is equal to 21.

Calculate X.

777xxxAAABBBCCCEEE

2

Step-by-step solution

To solve this problem, let's apply the following steps:

  • Step 1: Identify the formula for the area of a triangle, which is A=12×base×height A = \frac{1}{2} \times \text{base} \times \text{height} .
  • Step 2: Substitute the known values into the formula: 21=12×7×x 21 = \frac{1}{2} \times 7 \times x .
  • Step 3: Simplify and solve the equation for x x .

Now, let's work through each step more precisely:
Step 1: We're given the area formula as A=12×b×h A = \frac{1}{2} \times b \times h .
Step 2: Substitute in the known values: the area A=21 A = 21 , the base b=7 b = 7 , and the height h=x h = x , leading to the equation 21=12×7×x 21 = \frac{1}{2} \times 7 \times x .
Step 3: Solve for x x – first simplify the multiplication on the right: 21=72×x 21 = \frac{7}{2} \times x .
Step 4: To isolate x x , multiply both sides by 2 to get 42=7x 42 = 7x .
Step 5: Finally, divide both sides by 7 to solve for x x : x=427=6 x = \frac{42}{7} = 6 .

Therefore, the value of x x is 6 6 .

3

Final Answer

6

Key Points to Remember

Essential concepts to master this topic
  • Formula: Area = 12×base×height \frac{1}{2} \times \text{base} \times \text{height} for all triangles
  • Substitution: Replace known values: 21=12×7×x 21 = \frac{1}{2} \times 7 \times x
  • Verification: Check answer: 12×7×6=21 \frac{1}{2} \times 7 \times 6 = 21

Common Mistakes

Avoid these frequent errors
  • Forgetting to multiply by one-half in area formula
    Don't use Area = base × height = 7 × x = wrong answer! This gives x = 3 instead of 6. The triangle area formula always includes the fraction ½. Always use Area = 12×base×height \frac{1}{2} \times \text{base} \times \text{height} .

Practice Quiz

Test your knowledge with interactive questions

Angle A is equal to 30°.
Angle B is equal to 60°.
Angle C is equal to 90°.

Can these angles form a triangle?

FAQ

Everything you need to know about this question

Why do we use one-half in the triangle area formula?

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A triangle is half of a rectangle with the same base and height! If you imagine completing the triangle into a rectangle, the triangle takes up exactly half the space.

What if the base and height aren't clearly labeled?

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Look for the perpendicular height - it's the line that forms a 90° angle with the base. In this problem, x is drawn as a vertical line perpendicular to the horizontal base of 7.

Can I use a different side as the base?

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Yes! Any side can be the base, but then you need the perpendicular height to that side. The area will be the same no matter which base-height pair you choose.

How do I solve equations with fractions?

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When you have 21=7x2 21 = \frac{7x}{2} , multiply both sides by 2 first: 42=7x 42 = 7x . Then divide by 7 to get x = 6.

What units should my answer have?

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Since this is asking for a length (height), your answer should have the same units as the base. If no units are given, just write the number like 6.

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