Even for triangles shaped as below, the area of a triangle = base x height and half it.
The proof (but you don’t need it) is as follows:
Redraw the above diagram
Area of triangle `ABC=`Area triangle `ADB-`Area triangle `ADC`
Area of triangle `ABC=1/2timesDBtimesAD-1/2timesDCtimesAD`
You can remove `1/2AD`
Area of triangle `ABC=1/2AD(DB-DC)`
And because `DB-DC=CB`
We can now say:
Area of triangle `ABC=1/2ADtimesCB`
Slanting triangles are rare and it’s still best to think of areas of triangles in a rectangle.