Practise the method.Example 1A rectangle has width x and length 18−x metres. Its area is A(x)=x(18−x). Find A(5), in m2.Show worked answerAnswer: 65A(5)=5×(18−5)=65m2.Example 2For a rectangle with sides x and 20−x, which domain gives two positive side lengths?Show worked answerAnswer: 0<x<20Both x and 20−x must be positive. Hence 0<x<20.Example 3A ball's height is h(t)=−t2+14t, in metres. Apart from t=0, when is it at ground level?Show worked answerAnswer: 14Set h=0 and factor: t(14−t)=0. The positive landing time is 14 seconds.