11.
a)
[tex]a = x+4+x = 2x+4 = 2(x+2)\\h = 5\\P = 20\\x = ?\\\\\frac{1}{2}ah = P\\\\\frac{1}{2}\cdot2(x+2) \cdot5 = 20\\\\(x+2)\cdot5 = 20 \ \ /:5\\\\x+2 = 4\\\\x = 4-2\\\\\underline{x = 2}[/tex]
b)
[tex]3x-2+3x-2+2x+x+1+x+1 = 18\\\\10x -2 = 18\\\\10x = 18+2\\\\10x = 20 \ \ /:2\\\\\underline{x = 2}[/tex]
c)
[tex]P_1 = \frac{6+4}{2}\cdot x = 5x\\\\P_2 = 6(x+1) = 6x+6\\\\P_1+P_2 = 28\\\\\\5x+6x+6 = 28\\\\11x = 28-6\\\\11x = 22 \ \ /:11\\\\\underline{x = 2}[/tex]
