Assignment 6 solutions
Total=5 points
1. Find the tangent plane to the graph of the function
f
(
x,y
) = 2
π

3 cos 4
x
+ 3 sin 2
y
at the point (0
,π
).
Solution:
The tangent plane of
f
at a point (
a,b
) is given by the equation
z
=
f
(
a,b
) + grad
f
(
a,b
)
±
x

a
y

b
²
.
Here (
a,b
) = (0
,π
). The
x
partial derivative of
f
,
∂f
∂x
, is obtained by treating
y
as a constant
and taking the derivative with respect to
x
: here it is given by 0 + 12 sin 4
x
+ 0 = 12 sin 4
x
.
Similarly
∂f
∂y
= 6 cos 2
y
, and so
grad
f
=
±
∂f
∂x
,
∂f
∂y
²
= (12 sin 4
x,
6 cos 2
y
)
.
For the tangent plane (or linear approximation), we evaluate the gradient at the speciﬁed
point (0
,π
), that is, we plug in 0 for
x
and
π
for
y
:
grad
f
(
a,b
) = grad
f
(0
,π
) = (0
,
6)
.
So the tangent plane is given by
z
=
f
(
a,b
) + grad
f
(
a,b
)
±
x

a
y

b
²
=
f
(0
,π
) + (0
,
6)
±
x
y

π
²
=(2
π

3
*
1 + 3
*
0) + (0
,
6)
±
x
y

π
²
=(2
π

3) + 0
*
x
+ (6)(
y

π
)
=

4
π

3 + 6
y.
Alternatively we could have used the equivalent formula
z
=
f
(
a,b
) +
∂f
∂x
(
a,b
)(
x

a
) +
∂f
∂y
(
a,b
)(
y

b
)
for the tangent plane.
1