# Evaluating the area under a curve

Gold Member

## Homework Statement

The graph of ##y^2 + 2xy + 40 \mid x \mid =400## divides the plane into regions. The area of the bounded region is?

## Homework Equations

All relevant to calculus

## The Attempt at a Solution

My question is how to predict the graph without using a grapher here? I put x and y=0 to find the points where it cuts the axes (0,+-20) and (+-10,0). but I never could have guessed what it looks like without a computer (like in my exams)

Mark44
Mentor

## Homework Statement

The graph of ##y^2 + 2xy + 40 \mid x \mid =400## divides the plane into regions. The area of the bounded region is?

## Homework Equations

All relevant to calculus

## The Attempt at a Solution

My question is how to predict the graph without using a grapher here?
Split the equation into two parts: one equation in which x < 0 (resulting in ##y^2 + 2xy - 40x = 400##) and the other in which x >= 0 (resulting in ##y^2 + 2xy + 40x = 400##).
Krushnaraj Pandya said:
I put x and y=0 to find the points where it cuts the axes (0,+-20) and (+-10,0). but I never could have guessed what it looks like without a computer (like in my exams)

• Krushnaraj Pandya
LCKurtz
Homework Helper
Gold Member
Split the equation into two parts: one equation in which x < 0 (resulting in ##y^2 + 2xy - 40x = 400##) and the other in which x >= 0 (resulting in ##y^2 + 2xy + 40x = 400##).
That problem seems pretty tricky for an in class exam type question. It's easy enough to tell that the graph of each of these is a hyperbola by checking its discriminant. What isn't immediately obvious from either of these equations is that the hyperbolas are degenerate, meaning the graphs are actually the intersecting asymptotes of the would be hyperbolas. And given that they are translated and rotated from the "standard" equations, without a grapher of some sort it would take some significant time to work through the algebra and deduce that the two sections combine to give a parallelogram shaped region.

• Krushnaraj Pandya
ehild
Homework Helper
That problem seems pretty tricky for an in class exam type question. It's easy enough to tell that the graph of each of these is a hyperbola by checking its discriminant. What isn't immediately obvious from either of these equations is that the hyperbolas are degenerate, meaning the graphs are actually the intersecting asymptotes of the would be hyperbolas. And given that they are translated and rotated from the "standard" equations, without a grapher of some sort it would take some significant time to work through the algebra and deduce that the two sections combine to give a parallelogram shaped region.
It is easy to see that both expressions ##y^2 + 2xy - 40x - 400=0## and ##y^2 + 2xy + 40x - 400=0## can be written as difference of two squares. ##(y^2 + 2xy +x^2) -(x^2\pm40x+400)=0##.
Upon factorizing, any of the factors can be zero, which means four linear functions, enclosing a parallelogram.

• Krushnaraj Pandya
Gold Member
Split the equation into two parts: one equation in which x < 0 (resulting in ##y^2 + 2xy - 40x = 400##) and the other in which x >= 0 (resulting in ##y^2 + 2xy + 40x = 400##).
I worked that out, its the part that follows which is difficult

Gold Member
That problem seems pretty tricky for an in class exam type question. It's easy enough to tell that the graph of each of these is a hyperbola by checking its discriminant. What isn't immediately obvious from either of these equations is that the hyperbolas are degenerate, meaning the graphs are actually the intersecting asymptotes of the would be hyperbolas. And given that they are translated and rotated from the "standard" equations, without a grapher of some sort it would take some significant time to work through the algebra and deduce that the two sections combine to give a parallelogram shaped region.
That's high school in India...especially if you want to get into some of the best grad schools here, millions of students pay thousands of dollars to get tuition to learn 2 years worth of an engineering degree while still in the last year of school.
I figured this is a hyperbola, but as you said I need some insight to predict their shape

Gold Member
It is easy to see that both expressions ##y^2 + 2xy - 40x - 400=0## and ##y^2 + 2xy + 40x - 400=0## can be written as difference of two squares. ##(y^2 + 2xy +x^2) -(x^2\pm40x+400)=0##.
Upon factorizing, any of the factors can be zero, which means four linear functions, enclosing a parallelogram.
This is the most helpful insight I think, how did this occur to you so easily? and how can I develop this sort of intuition?

Gold Member
I think there must be an easier way to evaluate the area using calculus since this problem is given under calculating areas through integrals section of my textbook

ehild
Homework Helper
This is the most helpful insight I think, how did this occur to you so easily? and how can I develop this sort of intuition?
about 70 years practice Gold Member
It is easy to see that both expressions ##y^2 + 2xy - 40x - 400=0## and ##y^2 + 2xy + 40x - 400=0## can be written as difference of two squares. ##(y^2 + 2xy +x^2) -(x^2\pm40x+400)=0##.
Upon factorizing, any of the factors can be zero, which means four linear functions, enclosing a parallelogram.
Also, how can we figure out either these 4 functions or the vertices of the parallelogram? calculating the area without them isn't possible

Gold Member
about 70 years practice hahaha, doesn't seem achievable before my entrance exams. ehild
Homework Helper
Also, how can we figure out either these 4 functions or the vertices of the parallelogram? calculating the area without them isn't possible
Have you found the two square expressions?
y2+2xy+x2 is the square of???
x2±40x+400 is the square of????
How to factorize the difference of two squares ? a2-b2=?????

LCKurtz
Homework Helper
Gold Member
hahaha, doesn't seem achievable before my entrance exams. I guess you had better hurry! Good catch ehild. I didn't notice that. It's always "easy" after you see it. • Krushnaraj Pandya
Gold Member
Have you found the two square expressions?
y2+2xy+x2 is the square of???
x2±40x+400 is the square of????
How to factorize the difference of two squares ? a2-b2=?????
yes I did factorize it but I didn't see that its in the form a^2-b^2 (I'm half asleep right now...)
thank you very much for your help everyone :D
@LCKurtz I'll tell you if I won that race when my exams come, I'll have learned a lot regardless though

ehild
Homework Helper
yes I did factorize it but I didn't see that its in the form a^2-b^2 (I'm half asleep right now...)
thank you very much for your help everyone :D
@LCKurtz I'll tell you if I won that race when my exams come, I'll have learned a lot regardless though
Try to stay awake. You got (x+y)^2-(x±20)^2, did you not? a=x+y, b=x±20. Now factorize, what do you get?
The product is zero, so one factor has to be zero. That means 4 equations, 4 straight lines.

• Krushnaraj Pandya
Gold Member
Try to stay awake. You got (x+y)^2-(x±20)^2, did you not? a=x+y, b=x±20. Now factorize, what do you get?
The product is zero, so one factor has to be zero. That means 4 equations, 4 straight lines.
yes yes, I noted that right away after you hinted at it. I got 2x+y+20, 2x+y-20, y+20 and y-20 and drew the graph right away
:D

Gold Member
The area is 800 sq. units. Deemed correct by my textbook too.

ehild
Homework Helper
The area is 800 sq. units. Deemed correct by my textbook too.
It was very easy, wasn't it? Gold Member
It was very easy, wasn't it? yes, it really was.
Initially maths was one of my least favorite subjects since I always got stuck on problems like these and the education system here dictates that either the teachers pay attention only to those students who are super-smart and have the ability to score national ranks or do it just as a boring job- considering this, no one had the time or patience to point out simple things such as a^2-b^2 to me.
Now I really enjoy it and am improving, thanks to people like you :D

ehild
Homework Helper
yes, it really was.
Initially maths was one of my least favorite subjects since I always got stuck on problems like these and the education system here dictates that either the teachers pay attention only to those students who are super-smart and have the ability to score national ranks or do it just as a boring job- considering this, no one had the time or patience to point out simple things such as a^2-b^2 to me.
Now I really enjoy it and am improving, thanks to people like you :D

You also could have used the quadratic formula to solve ##y^2 + 2xy \pm 40x - 400=0## equation for y.
Do not forget: Maths is fun! But you need to practice. Gold Member
You also could have used the quadratic formula to solve ##y^2 + 2xy \pm 40x - 400=0## equation for y.
Do not forget: Maths is fun! But you need to practice. did it just to check, got the same answer, thank you :D

ehild
Homework Helper
did it just to check, got the same answer, thank you :D
Well done!

• Krushnaraj Pandya
Gold Member
Well done!
Thank you  LCKurtz