1. ## Right answer but...not quite ? (Quadratic)

So here is the question and the way I worked through it

The answer is 55.125 cm^2

I just feel as thought my method used to arrive at the answer is either wrong, or a misinterpretation of steps in order
.
It's really been bothering me, like there is an approach to the question, which is the correct procedure, and I haven't implemented it.

Take a look ladies & gents.

2. edit: I read the question again.

here is how I would do it:

you know that A = 1/2(21-h)*h = 10.5h - 0.5h^2

then dA/dh = 10.5 - h

when dA/dh = 0, h = 10.5

when h = 10.5, b = 10.5, so A = 55.125 cm^2

3. Originally Posted by harbottle
edit: read the question again.

here is how I would do it:

you know that A = 1/2(21-h)*h = 10.5h - 0.5h^2

then dA/dh = 10.5 - h

when dA/dh = 0, h = 10.5

when h = 10.5, b = 10.5
This is a good solution and leads to the same answer as the OP got. However, since the OP posted in the Pre-algebra and Algebra subforum it's likely that s/he hasn't studied calculus and therefore may not understand it. As a general rule, an expectation of replies to questions posted in this subforum would be that non-calculus methods be used.

@OP: You have made heavy weather of finding the zeros of the area function. Since the original expression was factorised, it ought to be evident that the zeroes are h = 21 and h = 0 using the null factor law (you might know this idea under a different name). The maximum turning point will therefore lie halfway between - at x = 10.5 etc.

The question appears to be dealt with satisfactorily so for various reasons I'm closing this thread.