a=jeden bok - x
b=drugi bok - 4x
obw prostokąta :
[tex]obw = 2a + 2b[/tex]
[tex]obw = 2 \times x + 2 \times 4x = 2x + 8x = 10x[/tex]
[tex]30cm = 10x \: \: \: \: | \div 10 \\ a = x = 3cm \\b = 4 \times 3 = 12cm[/tex]
pole prostokąta
[tex]p = a \times b[/tex]
[tex]p = 3 \times 12 = 36 {cm}^{2} [/tex]
