#26
Top 50 Mistake
Using slant height instead of perpendicular height
"The Slant Slip"
The Mistake in Action
Find the area of a triangle with base 8cm and slant side 10cm. The perpendicular height is 6cm.
Wrong: $A = \frac{1}{2} \times 8 \times 10 = 40\text{cm}^2$
Why It Happens
Students use the slant height (a side of the triangle) instead of the perpendicular height.
The Fix
The height in the area formula must be perpendicular (at 90°) to the base.
Look for:
- A line with a small square (indicating 90°)
- The word "perpendicular" or "vertical"
$$A = \frac{1}{2} \times 8 \times 6 = 24\text{cm}^2$$
Not: $\frac{1}{2} \times 8 \times 10 = 40$
Spot the Mistake
Can you identify where this student went wrong?
Triangle: base = 8cm, slant side = 10cm, perpendicular height = 6cm
$A = \frac{1}{2} \times 8 \times 10$
$= 40\text{cm}^2$
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: