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R=2/3h=2/3·a√3/2=a√3/3
r=1/3h=a√3/6
wzor na P=π(R²-r²)
12π=π(R²-r²) /:π
12=R²-r²
12=(a√3/3)²-(a√3/6)²
12=3a²/9-3a²/36
12=12a²/36-3a²/36
12=9a²/36
12=a²/4
a²=4·12
a²=48
a=√48=4√3 ---.bok tego Δ rownobocznego
zatem PΔ=a²·√3/4=(4√3)²·√3/4=48√3/4=12√3 [j²]