Question #153362

Shade the region of the xy-plane for which

a) (𝑥2+𝑦2−16)(𝑥2−4)≤0

b) (𝑦−𝑥)(𝑦2+𝑥3)>0


1
Expert's answer
2021-02-24T07:33:35-0500

Before shading this region, let's study geometrically to which region it corres[onds. The first inequality is achieved, when two expressions (x2+y216,x24x^2+y^2-16, x^2-4) have opposite signs. Now there is two cases : x2+y2160,x240x^2+y^2-16\leq 0, x^2-4\geq0, in this case the point should be in the circle centered at 0 of radius 4 (due to the first inequality) and it should have x2|x|\geq 2 due to the second case. The second case is when the point is outside the circle centered at 0 of radius 4, but it has x2|x|\leq 2. Thus we obtain a region like this :

We will study this region similarly to the previous one : this expression is positive if both the epressions yx,y2+x3y-x, y^2+x^3 are of the same sign. There is two cases: y>x,y2>x3y>x, y^2>-x^3 or y<x,y2<x3y<x, y^2<-x^3. The first case corresponds to points that are higher than the straight line y=xy=x, and for the points x<0x<0 we have additional condition y>x3|y|>\sqrt{-x}^3. In the other case we have points that are lower than y=xy=x, we can't have points x0x\geq0 (as we can't have y2<0y^2<0) and for x<0x<0 we have an additional condition y<x3|y|< \sqrt{-x}^3. Thus we obtain a region like this :

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