Can you solve these equations?

Algebra Level 3

x y + y z = 35 y z + x z = 32 x z + y x = 27 x 2 + y 2 + z 2 = ? \large\begin{aligned}\begin{aligned}xy+yz=&\ 35\\yz+xz=&\ 32\\xz+yx=&\ 27\\x^2+y^2+z^2=&\ ?\end{aligned}\end{aligned}


The answer is 50.

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4 solutions

Chew-Seong Cheong
Jun 23, 2015

{ x y + y z = 35 . . . ( 1 ) y z + z x = 32 . . . ( 2 ) z x + x y = 27 . . . ( 3 ) \begin{cases} xy+yz = 35 &...(1) \\ yz+zx = 32 &...(2) \\ zx+xy = 27 &...(3) \end{cases}

{ ( 1 ) + ( 2 ) + ( 3 ) : 2 ( x y + y z + z x ) = 94 x y + y z + z x = 47 . . . ( 4 ) ( 4 ) ( 1 ) : z x = 47 35 = 12 . . . ( 5 ) ( 4 ) ( 2 ) : x y = 47 32 = 15 . . . ( 6 ) ( 4 ) ( 3 ) : y z = 47 27 = 20 . . . ( 7 ) \Rightarrow \begin{cases} (1)+(2)+(3): & 2(xy+yz+zx) = 94 \\ & \Rightarrow xy+yz+zx = 47 & ... (4) \\ (4)-(1): & zx = 47-35 = 12 & ...(5) \\ (4)-(2): & xy = 47-32 = 15 & ...(6) \\ (4)-(3): & yz = 47-27 = 20 & ...(7) \end{cases}

{ ( 6 ) × ( 5 ) ( 7 ) : x y × z x y z = 15 × 12 20 x 2 = 9 x = ± 3 ( 6 ) : ± 3 y = 15 y = ± 5 ( 5 ) : ± 3 z = 12 z = ± 4 \Rightarrow \begin{cases} \dfrac{(6)\times (5)}{(7)}: & \dfrac {xy \times zx} {yz} = \dfrac {15\times 12} {20} & \Rightarrow x^2 = 9 & \Rightarrow x = \pm 3 \\ (6): & \pm 3 y = 15 & \Rightarrow y = \pm 5 \\ (5): & \pm 3 z = 12 & \Rightarrow z = \pm 4 \end{cases}

x 2 + y 2 + z 2 = 9 + 25 + 16 = 50 \Rightarrow x^2 + y^2 + z^2 = 9 + 25 + 16 = \boxed{50}

Moderator note:

You can simplify your work by a bit.

Hint: Add up the first three equations together.

Thanks. Yes, it is simpler. Edited.

Chew-Seong Cheong - 5 years, 11 months ago
Tasha Kim
Jun 22, 2015

First time writing a solution.. Hope this helps

Let (i) xy + yz = 35 (ii) yz + xz = 32 (iii) xz + yx = 27

Calculate (i) - (ii) + (iii) = (xy - xz) + (xz + yx) = 2xy = 30 xy = 15

(ii) - (iii) + (i) = (yz - yx) + (xy + yz) = 2yz = 40 yz = 20

(iii) - (i) + (ii) = (xz - yz) + (yz + xz) = 2xz = 24 xz = 12

By inserting x = 15/y and y = 20/z, you get z = 4

Then xz = 12 so x = 3

And xy = 15 so y = 5

Therefore, x^2 + y^2 + z^2 = 9 + 25 + 16

= 50

Good solution

Debmalya Mitra - 5 years, 11 months ago

The sum of 35 + 32 + 27 = 94 35+32+27=94 , which is 2 ( x y + y z + x z ) 2(xy+yz+xz) .

x y + y z + x z = 94 2 = 47 xy+yz+xz=\frac{94}{2} = 47

x z xz = 47 35 = 12 47-35=12

x y xy = 47 32 = 15 47-32=15

y z yz = 47 27 = 20 47-27=20

H C F ( 12 , 15 ) = 3 HCF(12,15) = 3

H C F ( 12 , 20 ) = 4 HCF(12,20) = 4

H C F ( 20 , 15 ) = 5 HCF(20,15) = 5

3 2 + 4 2 + 5 2 = 50 3^{2} + 4^{2} + 5^{2} = \boxed{50}

Moderator note:

What is the relevance of HCF \text{HCF} to this question? The question did not explicitly state that x , y , z x,y,z are integers.

Panya Chunnanonda
Jun 22, 2015

xy+ yz= 35...(1)
yz+ xz= 32...(2)
xz+ yx= 27...(3)
((1)+ (2)+ (3))/ 2...xy+ yz+ zx= 47..(4)
(4)- (1)...zx= 12...(5)
(4)- (2)...yx= 15...(6)
(4)- (3)...yz= 20...(7)
(5)* (6)* (7)...(xyz)^2= 12* 15* 20...or (xyz)= 3 4 5...(8)
(8)/ (5)...y= 5
(8)/ (6)...z= 4
(8)/ (7...x= 3
so...x^2+ y^2+ z^2= 9+ 25+ 16= 50





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