nomer 11 dan 12 apa ya jwban nya

Bab Bangun Datar
matematika SMP Kelas VIII

11] rhombus
3x + 1 = 10
3x = 10 - 1
x = 9/3
x = 3

12] trapezoid
titik FB tegak lurus dengan CD

gabung segitiga ADE dan BCF
menghasilkan segitiga ADC
karena ∠D = ∠C = 60°, maka ∠DAC = 60°, sehingga terbentuk segitiga sama sisi.

DE = 1/2 x (DC - AB)
DE = 1/2 x (28 - 18)
DE = 1/2 x 10
DE = 5 

maka, DC = 2 x DE = 2 x 5 = 10
karena ADC sama sisi, maka, semua sisinya 10, sehingga DC = AD = AC = 10

jadi, keliling = AD + AB + BC + CD
keliling = 10 + 18 + 10 + 28
keliling = 66

[tex]\displaystyle 12\\\text{DE}=\frac12(\text{DC}-\text{AB})\\\text{DE}=\frac12(28-18)\\\text{DE}=\frac12(10)\\\text{DE}=5\\\\\text{DE}=\text{AD}\cos\angle ADE\\5=\text{AD}\cos60^\circ\\5=\frac12\text{AD}\\10=\text{AD}=\text{BC}\\\\K_{\text{ABCD}}=\text{AB}+\text{CD}+\text{AD}+\text{BC}\\K_{\text{ABCD}}=18+28+10+10\\\boxed{\boxed{K_{\text{ABCD}}=66}}[/tex]

[tex]\displaystyle 11\\\text{CB}=\text{AB}\\3x+1=10\\3x=9\\\boxed{\boxed{x=3}}[/tex]