Why does my child use the whole base to find an isosceles triangle's height?
Your child uses the whole base of an isosceles triangle for its height, instead of half of it.
Why children do it
The base is the number given, so it's the one used. But the height cuts the triangle into two right-angled triangles, and each has half the base. With sides of 10 and a base of 12, it's 10² − 6² = 64, so the height is 8.
How to help at home
Ask them to draw the height in: it cuts the base in half, so each right-angled triangle has half.
The right way
An isosceles triangle has two sides of 10 cm and a base of 8 cm. How tall is it, to 1 decimal place?
- 10² − 4² = 84Half the base is 4. Take its square from the side's.
- 9.2√84 = 9.16..., which is 9.2 to 1 decimal place.
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