a)
[tex] \frac{(a + b) \times h}{2} = \frac{(9 + 19) \times 12}{2} = \frac{28 \times 12 }{2} = 14 \times 12 = 168 {dm}^{2} [/tex]
b)
[tex] \frac{(16 + 23) \times 20}{2} = \frac{39 \times 20}{2} = 39 \times 10 = 390 {cm}^{2} [/tex]
c)
[tex] \frac{(6 + 9) \times 5}{2} = \frac{15 \times 5}{2} = \frac{75}{2} = 37.5 {dm}^{2} [/tex]
