Hectare

The hectare (symbol ha, ˈhɛktɛər or ˈhɛktɑːr) is a unit of area defined as 10,000 square metres, which is primarily used in the measurement of land. In 1795, when the metric system was introduced, the are was defined as being 100 square metres and the hectare ('w:hecto-' + 'are') was thus 100 ares or 1/100 km².

Math

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  An illustration showing a hectare and an ares, both represented by squares with sides of 10 metres
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  100m by 100m
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  Trafalgar Square has an area of about one hectare