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Find the smallest value of an expression

(without using a calculator)


\bf√((x-8)^2+(y+4)^2) +|y+3x|

User Akim
by
7.3k points

1 Answer

6 votes

Explanation:

to eliminate the always positive absolute value at the end let's set y = -3x

that makes the absolute value term = 0 in all cases.

then we have

sqrt((x - 8)² + (-3x + 4)²)

the x-value that makes this a minimum result also makes

(x - 8)² + (-3x + 4)²

a minimum.

x² - 16x + 64 + 9x² - 24x + 16

10x² - 40x + 80

the x that makes this a minimum, also makes

x² - 4x + 8

a minimum.

we get the extreme value as zero of the first derivative :

0 = 2x - 4

4 = 2x

x = 2

so, we get the minimum of all these expressions for x = 2.

as y = -3x, we get y = -3×2 = -6.

the minimum value for the expression is with

x = 2

y = -6

so, the minimum value is

sqrt((2 - 8)² + (-6 + 4)²) + |-6 + 3×2| =

= sqrt((-6)² + (-2)²) + |-6 + 6| =

= sqrt(36 + 4) + 0 =

= sqrt(40) = 6.32455532...

User Samoka
by
8.1k points

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