MHB Finding area between 2 functions

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The discussion focuses on finding the area between two functions, specifically \(x = 2y^2\) and \(x = 1 - y\). The user correctly identifies the intersection points by solving the equation \(1 - y = 2y^2\), yielding \(y = 1\) and \(y = -1\), but is confused about the nature of these intersections. Another user clarifies that the correct intersection point for \(2y - 1 = 0\) is \(y = \frac{1}{2}\), while \(y = -1\) is also valid. Additionally, there is a separate area calculation involving \(x = 2 - y^2\) and \(x = y^2 - 2\), where the user arrives at an incorrect area result of \(4\sqrt{2}\) instead of the correct \(\frac{16\sqrt{2}}{3}\). The discussion highlights common pitfalls in calculating areas between curves and the importance of verifying intersection points.
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1.

I have to find the area between

$x = 2y^2$
and
$x = 1 - y$

I find the intersection points

$ 1 -y = 2y^2$

$2y^2 + y - 1= 0 $

$(2y - 1)(y + 1)= 0$

so y = 1 and -1

However, x = y - 1 is not a vertical line so I am not sure how 1 and -1 can be intersections. Also, when I plug these numbers into the original equation they don't equate, so what am I doing wrong here?

1. I have another problem for finding the area between

$x = 2 - y^2$

and
$x = y^2 - 2$

I found that the points of intersection are y =$ +/- \sqrt{2}$ and the greater function is $ x = 2 - y^2$ so

$\int_{-\sqrt{2}}^{\sqrt{2}} 4 - 2y^2 \,d$

then

$\left[4y - \frac{2y^3}{3}]\right]_1^3$

and

$[4\sqrt{2} - \frac{2\sqrt{8}}{3}] - [- 4 \sqrt{2} + \frac{4 \sqrt{8}}{3}]$

and then

$4\sqrt{2}$

However the answer is $\frac{16\sqrt{2}}{3}$
 
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tmt said:
1.

I have to find the area between

$x = 2y^2$
and
$x = 1 - y$

I find the intersection points

$ 1 -y = 2y^2$

$2y^2 + y - 1= 0 $

$(2y - 1)(y + 1)= 0$

Correct up to here. Now notice that $\displaystyle \begin{align*} 2y - 1 = 0 \implies 2y = 1 \implies y = \frac{1}{2} \end{align*}$. The other solution y = -1 is fine...
 
There are probably loads of proofs of this online, but I do not want to cheat. Here is my attempt: Convexity says that $$f(\lambda a + (1-\lambda)b) \leq \lambda f(a) + (1-\lambda) f(b)$$ $$f(b + \lambda(a-b)) \leq f(b) + \lambda (f(a) - f(b))$$ We know from the intermediate value theorem that there exists a ##c \in (b,a)## such that $$\frac{f(a) - f(b)}{a-b} = f'(c).$$ Hence $$f(b + \lambda(a-b)) \leq f(b) + \lambda (a - b) f'(c))$$ $$\frac{f(b + \lambda(a-b)) - f(b)}{\lambda(a-b)}...

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