# Change of variable

by squenshl
Tags: variable
 P: 324 How do I evaluate the triple integral $$\int\int\int_G$$ x+y+z dV using a suitable change of variable where G is the region 0 $$\leq$$ x+y $$\leq$$ 1, 2 $$\leq$$ y+z $$\leq$$ 3, 4 $$\leq$$ x+z $$\leq$$ 5. I know to let u = x+y, v = y+z, w = x+z and I end up with the det(jac) = |2| $$\Rightarrow$$ 1/det(jac) = |1/2|. But I'm stuck after that. Help.
PF Patron
HW Helper
Sci Advisor
Thanks
P: 25,487
Hi squenshl!
 Quote by squenshl How do I evaluate the triple integral $$\int\int\int_G$$ x+y+z dV using a suitable change of variable where G is the region.
Well, you've got the bounds, and you know how to rewrite the dV (from the Jacobian), so all you need is to rewrite x+y+z in terms of u v and w, which is … ?
 PF Patron HW Helper Sci Advisor Thanks P: 11,935 Hint: What does u+v+w equal, in terms of x+y+z?
P: 324

## Change of variable

u+v+w = 2x+2y+2z = 2(x+y+z),
$$\Rightarrow$$ x+y+z = (u+v+w)/2.
Then just chuck that in. Is that right. Thanks.
PF Patron
HW Helper
Sci Advisor
Thanks
P: 25,487
 Quote by squenshl x+y+z = (u+v+w)/2.
Woohoo!
 P: 324 Cheers.

 Related Discussions General Math 8 Differential Equations 4 Introductory Physics Homework 4 Introductory Physics Homework 0 Calculus 1