Question

Find the linear approximation of the function f(x, y, z) = x2 + y2 + z2 at (4, 4, 7) and use it to approximate the number 4.022 + 3.992 + 6.972 . (Round your answer to five decimal places.) f(4.02, 3.99, 6.97) ≈

Answer 1

Answer:

80.66

Step-by-step explanation:

$L\left( x \right) = f\left( a \right) + f'\left( a \right)\left( {x - a} \right)$

$f(x, y, z) = x^2+y^2+z^2$

Since we have three variables,

$L(x, y,z) = f(a, b,c) + f_x (a, b,c) (x - a) + f_y (a, b,c) (y - b) +f_z(a,b,c)(z-c)$

$f(4, 4, 7) = 4^2+4^2+7^2=81$

$f_x (a, b,c)=2x=2*4=8\\f_y (a, b,c)=2y=2*4=8\\ f_z(a,b,c)=2z=2*7=14$

Therefore:

$L(x, y,z) = 81+ 8(x - 4) + 8 (y - 4) +14(z-7)$

Using the above:

f(4.02, 3.99, 6.97) =81+ 8(4.02 - 4) + 8 (3.99 - 4) +14(6.97-7)=80.66.

The approximation of  $4.02^2 + 3.99^2 + 6.97^2$ is 80.66000.