Answer:
[tex]let \: area \: be \: a, \: height \: be \: h, \: base \: be \: b \\ a = \frac{1}{2} bh \\ a + \delta \: a = \frac{1}{2} \times (b + \delta \: b)(h + \delta \: h) \\ a + \delta \: a = \frac{1}{2} (bh + b\delta \: h + h\delta \: b + 0) \\ \delta \: a = \frac{1}{2} (b\delta \: h + h\delta \: b) \\ \frac{\delta \: a}{a} = \frac{b\delta \: h + h\delta \: b}{bh} \\ \frac{\delta \: a}{a} = | \frac{\delta \: h}{h} | + | \frac{\delta \: b}{b} | \\ \frac{\delta \: a}{ 90} = \frac{0.5}{15} + \frac{0.5}{6} \\ \delta \: a = 10.5 \: {cm}^{2} [/tex]
error is 10.5
