Max/Min problem help (1 Viewer)

[SophJI]

i had a question like

A= 2x^2+1/2(pie)r^3+r^2 divide by 2 ( not 100% correct, it was similar)

(It was the area of a rectangle and a semi cicle)

and they asked me show that it's a maximum

when i differentiate that does the pie disappear? or does it stay??

i am so confused can anyone help?

+the attatchment is my drawing of the question they gave us

 

[tommykins]

pi is a number and is a constant, so it has to stay.

 

[spagbowl]

how can u differentiate something with two variables. i dont know whats going on either. did they give u another equation which u can solve for r or x and get rid of one of them to create an equation with just one variable. ohhh i am so screwed for maths half yearly...

 

[SophJI]

how can u differentiate something with two variables. i dont know whats going on either. did they give u another equation which u can solve for r or x and get rid of one of them to create an equation with just one variable. ohhh i am so screwed for maths half yearly...

This is was a question from my half yearly paper today
they gave me an area of a rectangle+semi circle and asked me to show the maximum
i was confused too as well and i think i just stuffed up a 6 mark question
CRAP

 

[Bacchus]

Are you saying that

r = pi?

Because that question sounds a bit retarded considering that there are 2 variables.

Since pi is a constant, it remains there.

E.g.

y = 4x where 4 is the constant
dy/dx = 4

another e.g.

y = pi x
dy/dx = pi

EDIT: JUST POST UP THE WHOLE QUESTION.

 

[GUSSSSSSSSSSSSS]

well actually picturing the question in my head

ur gonna be able to find a relationship between x and r WHICH IS NECESSARY

(however thats only if the semicircle takes up a FULL side of the rectangle)


also good luck wiv ur otha exams XDD

 

[Jimmy Marshman]

Did they give you the exact perimeter or area? cause you will need to use simultanous equations to find wat x or r equals in relation to the other

 

[SophJI]

well actually picturing the question in my head

ur gonna be able to find a relationship between x and r WHICH IS NECESSARY

(however thats only if the semicircle takes up a FULL side of the rectangle)


also good luck wiv ur otha exams XDD

What are you on about?
Your making things so complicated for my brain..can you just tell me if i need to remove the pie or not

and thanks good luck to you as well

 

[SophJI]

Did they give you the exact perimeter or area? cause you will need to use simultanous equations to find wat x or r equals in relation to the other

What???
They gave me an Area of rectangle+semicircle
and told me to find the maximum

why do you need simultanous questons???

 

[GUSSSSSSSSSSSSS]

What are you on about?
Your making things so complicated for my brain..can you just tell me if i need to remove the pie or not

and thanks good luck to you as well

lol sorry

YOU DO NOT HAVE TO REMOVE THE PI AS IT IS A CONSTANT


however that equation cannot be differentiated unless either the x or r is ALSO kept constant


lol ty

 

[Bacchus]

What???
They gave me an Area of rectangle+semicircle
and told me to find the maximum

why do you need simultanous questons???

hey, we cannot really help you if you don't know the question

 

[GUSSSSSSSSSSSSS]

well looking at ur picture

u still need something with which to connect "r" and "x"
either like a perimeter
or perhaps saying the long side is twice the short side
or something like that

COS OTHERWISE YOU ARE STUCK WITH AN EQUATION WITH 2 VARIABLES TO DIFFERENTIATE W.R.T. and that is something you cant do..