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O=2a+2b
wiadomo ze
a=30%·O
9=0,3·O
O=9:0,3
O=30
podstawiamy:
30=2·9+2b
30=18+2b
30-18=2b
12=2b /:2
b=6cm
pole prostokata
P=a·b=9cm·6cm=54cm²