Decide if the following statement is valid or invalid.
If the base of a triangle is 3 and its
height is 4, then its area is 6. This
triangle has an area of 6, so its
base is 3 and its height is 4.

Step-by-step explanation:

the area of a triangle is base times height divided by 2.

A = (b × h) / 2

so, when b = 3 and h = 4, then

A = (3 × 4) / 2 = 12 / 2 = 6

correct.

now the other way around

we know the area (6).

=> 6 = (b × h) / 2

=> 12 = b × h

well, b=3, h=4 is one possible solution for this.

but so is e.g. b=2, h=6, or b=12, h=1, or ...

and that is just the natural numbers. then there are rational numbers and so on.

therefore there is an infinite set of possible (b, h) pairs that satisfy the equation 12 = b × h.

therefore the statement that we can conclude out of a given triangle area of 6 that b must be then 3 and h must be 4 is wrong.

simple - it is one equation with 2 variables and has therefore usually not just one solution.