Calculus- Early Transcendentals, 2021a

502 Multiple Integration
Figure 14.11: Small parallelograms at points of tangency.
Now recall a curious fact: the area of a parallelogram can be computed as the cross product of two
vectors. We simply need to acquire two vectors, parallel to the sides of the parallelogram and with lengths
to match. But this is easy: in the x-direction we use the tangent vector we already know, namely 〈1,0, f x 〉
and multiply by Δx to shrink it to the right size: 〈Δx,0, f x Δx〉. Inthey-direction we do
√
the same thing and
get 〈0,Δy, f y Δy〉. The cross product of these vectors is 〈 f x , f y ,−1〉ΔxΔy with length fx 2 + fy 2 + 1ΔxΔy,
the area of the parallelogram. Now we add these up and take the limit, to produce the integral
∫ x1
∫ y1 √
fx 2 + f y 2 + 1dydx.
As before, the limits need not be constant.
Example 14.9: Surface Area of a Hemisphere
Find the area of the hemisphere z = √ 1 − x 2 − y 2 .
x 0
y 0
Solution. We compute the derivatives
and then the area is
f x =
∫ 1 ∫ √ √
1−x 2
−1 − √ 1−x 2
−x
−y
√ f x = √
1 − x 2 − y 2 1 − x 2 − y , 2
1 − x 2 − y 2 + y 2
1 − x 2 − y 2 + 1dydx.
This is a bit on the messy side, but we can use polar coordinates:
∫ 2π ∫ √ 1 1
1 − r 2 rdrdθ.
0
0
This integral is improper, since the function is undefined at the limit 1. We therefore compute
∫ √ a 1
√
lim
rdr= lim
a→1 − 0 1 − r2 − 1 − a 2 + 1 = 1,
a→1 −
x 2