Choose one base, then match its height

Any side can be the base, but the height must be measured perpendicular to that same base or its extended line. A slanted side is not the height unless it meets the base at a right angle.

Calculate triangle area

Choose a base.
Identify the opposite vertex.
Measure the perpendicular height.
Multiply base by height.
Divide by 2.
Label the result in square units.

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Base-height area formula

Keep the base-height pair together; changing the base changes which height belongs with it.

Area

A = \frac{b \times h}{2}

Paired base and height

A = \frac{b_{chosen} \times h_{perpendicular}}{2}

Unit example

\frac{12 \times 7}{2} = 42 {cm}^{2}

Slanted-side caution

slanted side \neq height unless perpendicular to the base

Common checks

Look for a right-angle marker.
Use the same length units for base and height.
Report area in square units.
Do not reuse a side length as height by sight.

FAQ

Can any triangle side be the base?

Yes. Any side can be the base, but the height must be the perpendicular distance from that base to the opposite vertex.

Can I use a slanted side as the height?

Only if that slanted side is perpendicular to the chosen base. If it leans along the triangle edge without forming a right angle to the base, it is not the height.