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3 years ago

Find the value of x³+ y³ + z³ – 3xyz if x² + y² + z² = 83 and x + y + z = 15

Mathematics

1 answer:

raketka [301]3 years ago

4 0

identity used is

x³ + y³+ z³– 3xyz = (x + y + z) (x² + y² + z² – xy – yz – zx).

then use

(x+y+z)² = x²+y²+z²+2(xy+yz+xz)

225= 83 + 2(xy+yz+xz)

xy+yz+xz = (225-83)/2

xy+yz+xz= 142/2

xy+yz+xz= 71

now use identity

x³ + y³+ z³– 3xyz = (x + y + z) (x² + y² + z² – xy – yz – zx).

now

x³ + y³+ z³– 3xyz = 15 (83 – xy – yz – zx).

= 15[83 - (71)]

= 15×12

=180

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we need to figure out slant length first

recall the formula of surface area

$\displaystyle A_{\text{surface}}=B+\dfrac{1}{2}\times P \times s$

where B stands for Base area

and P for Base Parimeter

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now we need our algebraic skills to figure out s

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reduce fraction:

$\sf\displaystyle \: 64+\dfrac{1}{ \cancel{ \: 2}}\times \cancel{32} \: ^{16} \times s = 144 \\ 64 + 16 \times s = 144$

simplify multiplication:

$\displaystyle \: 16s + 64 = 144$

cancel 64 from both sides;

$\displaystyle \: 16s = 80$

divide both sides by 16:

$\displaystyle \: \therefore \: s = 5$

now we'll use Pythagoras theorem to figure out height

according to the theorem

$\displaystyle \: {h}^{2} + (\frac{l}{2} {)}^{2} = {s}^{2}$

substitute the value of l and s:

$\displaystyle \: {h}^{2} + (\frac{8}{2} {)}^{2} = {5}^{2}$

simplify parentheses:

$\displaystyle \: {h}^{2} + (4 {)}^{2} = {5}^{2}$

simplify squares:

$\displaystyle \: {h}^{2} + 16 = 25$

cancel 16 from both sides:

$\displaystyle \: {h}^{2} = 9$

square root both sides:

$\displaystyle \: \therefore \: {h}^{} = 3$

recall the formula of a square pyramid

$\displaystyle V_{pyramid}=\dfrac{1}{3}\times A\times h$

where A stands for Base area (l²)

substitute the value of h and l:

$\sf\displaystyle V_{ \text{pyramid}}=\dfrac{1}{3}\times \{8 \times 8 \}\times 3$

simplify multiplication:

$\sf\displaystyle V_{ \text{pyramid}}=\dfrac{1}{3}\times 64\times 3$

reduce fraction:

$\sf\displaystyle V_{ \text{pyramid}}=\dfrac{1}{ \cancel{ 3 \: }}\times 64\times \cancel{ \: 3}$

hence,

$\sf\displaystyle V_{ \text{pyramid}}= 64 \: {cm}^{3}$

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Find the value of x³+ y³ + z³ – 3xyz if x² + y² + z² = 83 and x + y + z = 15​

Find the value of x³+ y³ + z³ – 3xyz if x² + y² + z² = 83 and x + y + z = 15