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a. Lebar^2 = Diagonal^2 - Panjang^2
L^2 = 25^2 - 15^2
L^2 = 625 - 225
L^2 = 400
[tex]l = \sqrt{400} \\ l = 20 \: cm[/tex]
Keliling persegi panjang
= 2(p + l)
= 2(25 cm + 20 cm)
= 2(45 cm)
= 90 cm

Luas persegi panjang
= p × l
= 25 cm × 20 cm
= 500 cm^2

b. Jika x = sisi persegi

Maka,
Diagonal^2 = x^2 + x^2
[tex] {10 \sqrt{2} }^{2} =2 {x}^{2} \\ 100 \times 2 = 2{x}^{2} \\ 100 = {x}^{2} \\ x = \sqrt{100} \\ x = 10 \: cm[/tex]

Luas persegi
= sisi × sisi
= x × x
= 10 cm × 10 cm
= 100 cm^2

Keliling persegi
= 4 × sisi
= 4 × x
= 4 × 10 cm
= 40 cm