QUESTION 4.1
As we know, the distance between points A ( x A , y A , z A ) A\left(x_A,y_A,z_A\right) A ( x A , y A , z A ) and B ( x B , y B , z B ) B\left(x_B,y_B,z_B\right) B ( x B , y B , z B ) given by their Cartesian coordinates can be calculated by the formula
A B = ( x A − x B ) 2 + ( y A − y B ) 2 + ( z A − z B ) 2 AB=\sqrt{\left(x_A-x_B\right)^2+\left(y_A-y_B\right)^2+\left(z_A-z_B\right)^2} A B = ( x A − x B ) 2 + ( y A − y B ) 2 + ( z A − z B ) 2
In our case,
{ A ( − 1 , 0 , 1 ) B ( 0 , − 2 , 3 ) C ( − 4 , 4 , 1 ) ⟶ { A B = ( 0 + 1 ) 2 + ( 0 + 2 ) 2 + ( 3 − 1 ) 2 B C = ( 0 + 4 ) 2 + ( 4 + 2 ) 2 + ( 3 − 1 ) 2 A C = ( − 1 + 4 ) 2 + ( 4 − 0 ) 2 + ( 1 − 1 ) 2 ⟶ { A B = 3 B C = 56 ≡ 2 14 A C = 5 \left\{\begin{array}{l}
A\left(-1,0,1\right)\\
B\left(0,-2,3\right)\\
C\left(-4,4,1\right)\\
\end{array}\right.\longrightarrow\\[0.3cm]
\left\{\begin{array}{l}
AB=\sqrt{\left(0+1\right)^2+\left(0+2\right)^2+\left(3-1\right)^2}\\[0.3cm]
BC=\sqrt{\left(0+4\right)^2+\left(4+2\right)^2+\left(3-1\right)^2}\\[0.3cm]
AC=\sqrt{\left(-1+4\right)^2+\left(4-0\right)^2+\left(1-1\right)^2}\\
\end{array}\right.\longrightarrow\\[0.3cm]
\left\{\begin{array}{l}
AB=3\\[0.3cm]
BC=\sqrt{56}\equiv2\sqrt{14}\\[0.3cm]
AC=5\\
\end{array}\right. ⎩ ⎨ ⎧ A ( − 1 , 0 , 1 ) B ( 0 , − 2 , 3 ) C ( − 4 , 4 , 1 ) ⟶ ⎩ ⎨ ⎧ A B = ( 0 + 1 ) 2 + ( 0 + 2 ) 2 + ( 3 − 1 ) 2 BC = ( 0 + 4 ) 2 + ( 4 + 2 ) 2 + ( 3 − 1 ) 2 A C = ( − 1 + 4 ) 2 + ( 4 − 0 ) 2 + ( 1 − 1 ) 2 ⟶ ⎩ ⎨ ⎧ A B = 3 BC = 56 ≡ 2 14 A C = 5
ANSWER
A B = 3 B C = 2 14 A C = 5 \boxed{AB=3\quad\quad BC=2\sqrt{14}\quad\quad AC=5} A B = 3 BC = 2 14 A C = 5
QUESTION 4.2
As in the previous problem, we calculate the lengths of the sides of the triangle with vertices P ( 3 , 1 , − 1 ) P\left(3,1,-1\right) P ( 3 , 1 , − 1 ) , Q ( 2 , 0 , 3 ) Q\left(2,0,3\right) Q ( 2 , 0 , 3 ) and R ( 1 , 1 , 1 ) R\left(1,1,1\right) R ( 1 , 1 , 1 ) , then check the fulfillment of the Pythagorean theorem.
{ P ( 3 , 1 , − 1 ) Q ( 2 , 0 , 3 ) R ( 1 , 1 , 1 ) ⟶ { P Q = ( 3 − 2 ) 2 + ( 1 − 0 ) 2 + ( 3 + 1 ) 2 Q R = ( 2 − 1 ) 2 + ( 1 − 0 ) 2 + ( 3 − 1 ) 2 P R = ( 3 − 1 ) 2 + ( 1 − 1 ) 2 + ( 1 + 1 ) 2 ⟶ { P Q = 18 ≡ 3 2 Q R = 6 P R = 8 ≡ 2 2 \left\{\begin{array}{l}
P\left(3,1,-1\right)\\
Q\left(2,0,3\right)\\
R\left(1,1,1\right)\\
\end{array}\right.\longrightarrow\\[0.3cm]
\left\{\begin{array}{l}
PQ=\sqrt{\left(3-2\right)^2+\left(1-0\right)^2+\left(3+1\right)^2}\\[0.3cm]
QR=\sqrt{\left(2-1\right)^2+\left(1-0\right)^2+\left(3-1\right)^2}\\[0.3cm]
PR=\sqrt{\left(3-1\right)^2+\left(1-1\right)^2+\left(1+1\right)^2}\\
\end{array}\right.\longrightarrow\\[0.3cm]
\left\{\begin{array}{l}
PQ=\sqrt{18}\equiv3\sqrt{2}\\[0.3cm]
QR=\sqrt{6}\\[0.3cm]
PR=\sqrt{8}\equiv2\sqrt{2}\\
\end{array}\right. ⎩ ⎨ ⎧ P ( 3 , 1 , − 1 ) Q ( 2 , 0 , 3 ) R ( 1 , 1 , 1 ) ⟶ ⎩ ⎨ ⎧ PQ = ( 3 − 2 ) 2 + ( 1 − 0 ) 2 + ( 3 + 1 ) 2 QR = ( 2 − 1 ) 2 + ( 1 − 0 ) 2 + ( 3 − 1 ) 2 PR = ( 3 − 1 ) 2 + ( 1 − 1 ) 2 + ( 1 + 1 ) 2 ⟶ ⎩ ⎨ ⎧ PQ = 18 ≡ 3 2 QR = 6 PR = 8 ≡ 2 2
As P Q > Q R > P R PQ>QR>PR PQ > QR > PR , only P Q PQ PQ can be the hypotenuse, then
P Q 2 ∧ Q R 2 + P R 2 ( 18 ) 2 ∧ ( 6 ) 2 + ( 8 ) 2 18 ≠ 6 + 8 PQ^2\land QR^2+PR^2\\[0.3cm]
\left(\sqrt{18}\right)^2 \land \left(\sqrt{6}\right)^2+\left(\sqrt{8}\right)^2\\[0.3cm]
18\neq6+8 P Q 2 ∧ Q R 2 + P R 2 ( 18 ) 2 ∧ ( 6 ) 2 + ( 8 ) 2 18 = 6 + 8
Conclusion,
Δ P Q R − not right triangle \boxed{\Delta PQR-\text{not right triangle}} Δ PQR − not right triangle
QUESTION 4.3
Let u → = ( 0 , 1 , 1 ) \overrightarrow{u}=\left(0,1,1\right) u = ( 0 , 1 , 1 ) , v → = ( 2 , 2 , 0 ) \overrightarrow{v}=\left(2,2,0\right) v = ( 2 , 2 , 0 ) and w → = ( − 1 , 1 , 0 ) \overrightarrow{w}=\left(−1,1,0\right) w = ( − 1 , 1 , 0 )
(a) As we know, if the angle between vectors a → \overrightarrow{a} a and b → \overrightarrow{b} b is 9 0 ∘ 90^\circ 9 0 ∘ , then a → ⋅ b → = 0 \overrightarrow{a}\cdot\overrightarrow{b}=0 a ⋅ b = 0 .
In our case,
{ u → = ( 0 , 1 , 1 ) v → = ( 2 , 2 , 0 ) w → = ( − 1 , 1 , 0 ) ⟶ { u → ⋅ v → = 0 ⋅ 2 + 1 ⋅ 2 + 1 ⋅ 0 = 2 ≠ 0 v → ⋅ w → = − 1 ⋅ 2 + 1 ⋅ 2 + 0 ⋅ 0 = 0 u → ⋅ w → = − 1 ⋅ 0 + 1 ⋅ 1 + 1 ⋅ 0 = 1 ≠ 0 \left\{\begin{array}{l}
\overrightarrow{u}=\left(0,1,1\right)\\[0.3cm]
\overrightarrow{v}=\left(2,2,0\right)\\[0.3cm]
\overrightarrow{w}=\left(−1,1,0\right)\\
\end{array}\right.\longrightarrow\\[0.3cm]
\left\{\begin{array}{l}
\overrightarrow{u}\cdot\overrightarrow{v}=0\cdot2+1\cdot2+1\cdot0=2\neq0\\[0.3cm]
\overrightarrow{v}\cdot\overrightarrow{w}=-1\cdot2+1\cdot2+0\cdot0=0\\[0.3cm]
\overrightarrow{u}\cdot\overrightarrow{w}=-1\cdot0+1\cdot1+1\cdot0=1\neq0\\
\end{array}\right. ⎩ ⎨ ⎧ u = ( 0 , 1 , 1 ) v = ( 2 , 2 , 0 ) w = ( − 1 , 1 , 0 ) ⟶ ⎩ ⎨ ⎧ u ⋅ v = 0 ⋅ 2 + 1 ⋅ 2 + 1 ⋅ 0 = 2 = 0 v ⋅ w = − 1 ⋅ 2 + 1 ⋅ 2 + 0 ⋅ 0 = 0 u ⋅ w = − 1 ⋅ 0 + 1 ⋅ 1 + 1 ⋅ 0 = 1 = 0
Conclusion,
v → ⋅ w → = 0 ⟶ v → ⊥ w → \boxed{\overrightarrow{v}\cdot\overrightarrow{w}=0\longrightarrow\overrightarrow{v}\perp\overrightarrow{w}} v ⋅ w = 0 ⟶ v ⊥ w
(b) As we know, the angle between the vectors can be found by the formula
cos ϕ = u → ⋅ w → ∥ u → ∥ ⋅ ∥ w → ∥ = 1 0 2 + 1 2 + 1 2 ⋅ ( − 1 ) 2 + 1 2 + 0 2 cos ϕ = 1 2 ⋅ 2 = 1 2 ⟶ ϕ = 6 0 ∘ \cos\phi=\frac{\overrightarrow{u}\cdot\overrightarrow{w}}{\left\|\overrightarrow{u}\right\|\cdot\left\|\overrightarrow{w}\right\|}=\frac{1}{\sqrt{0^2+1^2+1^2}\cdot\sqrt{(-1)^2+1^2+0^2}}\\[0.3cm]
\cos\phi=\frac{1}{\sqrt{2}\cdot\sqrt{2}}=\frac{1}{2}\longrightarrow\boxed{\phi=60^\circ} cos ϕ = ∥ ∥ u ∥ ∥ ⋅ ∥ ∥ w ∥ ∥ u ⋅ w = 0 2 + 1 2 + 1 2 ⋅ ( − 1 ) 2 + 1 2 + 0 2 1 cos ϕ = 2 ⋅ 2 1 = 2 1 ⟶ ϕ = 6 0 ∘
QUESTION 4.5
Let n → = ( x , y , z ) \overrightarrow{n}=(x,y,z) n = ( x , y , z ) is orthogonal to both u → = ( 0 , − 1 , − 1 ) \overrightarrow{u}=(0,-1,-1) u = ( 0 , − 1 , − 1 ) and v → = ( 1 , 0 , − 1 ) \overrightarrow{v}=(1,0,-1) v = ( 1 , 0 , − 1 ) , then
n → ⋅ u → = 0 \overrightarrow{n}\cdot\overrightarrow{u}=0 n ⋅ u = 0 and n → ⋅ v → = 0 \overrightarrow{n}\cdot\overrightarrow{v}=0 n ⋅ v = 0 .
{ n → ⋅ u → = 0 ⋅ x + ( − 1 ) ⋅ y + ( − 1 ) ⋅ z = − y − z = 0 n → ⋅ v → = 1 ⋅ x + 0 ⋅ y + ( − 1 ) ⋅ z = x − z = 0 { y = − z x = z z = C ⟶ n → = ( C , − C , C ) \left\{\begin{array}{l}
\overrightarrow{n}\cdot\overrightarrow{u}=0\cdot x+(-1)\cdot y+(-1)\cdot z=-y-z=0\\[0.3cm]
\overrightarrow{n}\cdot\overrightarrow{v}=1\cdot x+0\cdot y+(-1)\cdot z=x-z=0\\
\end{array}\right.\\[0.3cm]
\left\{\begin{array}{l}
y=-z\\x=z\\z=C
\end{array}\right.\longrightarrow\boxed{\overrightarrow{n}=\left(C,-C,C\right)} ⎩ ⎨ ⎧ n ⋅ u = 0 ⋅ x + ( − 1 ) ⋅ y + ( − 1 ) ⋅ z = − y − z = 0 n ⋅ v = 1 ⋅ x + 0 ⋅ y + ( − 1 ) ⋅ z = x − z = 0 ⎩ ⎨ ⎧ y = − z x = z z = C ⟶ n = ( C , − C , C )
It remains to make the found vector unit, that is ∥ n → ∥ = 1 \left\|\overrightarrow{n}\right\|=1 ∥ ∥ n ∥ ∥ = 1 . In our case,
∥ n → ∥ = C 2 + ( − C ) 2 + C 2 = C 3 = 1 → C = 1 3 \left\|\overrightarrow{n}\right\|=\sqrt{C^2+(-C)^2+C^2}=C\sqrt{3}=1\to\boxed{C=\frac{1}{\sqrt{3}}} ∥ ∥ n ∥ ∥ = C 2 + ( − C ) 2 + C 2 = C 3 = 1 → C = 3 1
Conclusion,
n → = ( 1 3 , − 1 3 , 1 3 ) \boxed{\overrightarrow{n}=\left(\frac{1}{\sqrt{3}},\frac{-1}{\sqrt{3}},\frac{1}{\sqrt{3}}\right)} n = ( 3 1 , 3 − 1 , 3 1 )
QUESTION 4.4
Let x < 0 x < 0 x < 0 . Find the vector n → = ( x , y , z ) \overrightarrow{n}=\left(x,y,z\right) n = ( x , y , z ) that is orthogonal to all three vectors u → = ( 1 , 1 , − 2 ) \overrightarrow{u}=\left(1,1,-2\right) u = ( 1 , 1 , − 2 ) , v → = ( − 1 , 2 , 0 ) \overrightarrow{v}=\left(−1,2,0\right) v = ( − 1 , 2 , 0 ) and w → = ( − 1 , 0 , 1 ) \overrightarrow{w}=\left(−1,0,1\right) w = ( − 1 , 0 , 1 ) . Then,
{ n → ⋅ u → = 1 ⋅ x + 1 ⋅ y + ( − 2 ) ⋅ z = x + y − 2 z = 0 n → ⋅ v → = − 1 ⋅ x + 2 ⋅ y + 0 ⋅ z = − x + 2 y = 0 n → ⋅ w → = − 1 ⋅ x + 0 ⋅ y + 1 ⋅ z = − x + z = 0 { 2 y + y − 2 ⋅ ( 2 y ) = 0 → − y = 0 x = 2 y z = x = 2 y ⟶ n → = ( 0 , 0 , 0 ) \left\{\begin{array}{l}
\overrightarrow{n}\cdot\overrightarrow{u}=1\cdot x+1\cdot y+(-2)\cdot z=x+y-2z=0\\[0.3cm]
\overrightarrow{n}\cdot\overrightarrow{v}=-1\cdot x+2\cdot y+0\cdot z=-x+2y=0\\[0.3cm]
\overrightarrow{n}\cdot\overrightarrow{w}=-1\cdot x+0\cdot y+1\cdot z=-x+z=0\\
\end{array}\right.\\[0.3cm]
\left\{\begin{array}{l}
2y+y-2\cdot(2y)=0\rightarrow-y=0\\[0.3cm]
x=2y\\[0.3cm]
z=x=2y\\
\end{array}\right.\longrightarrow\\[0.3cm]
\boxed{\overrightarrow{n}=(0,0,0)} ⎩ ⎨ ⎧ n ⋅ u = 1 ⋅ x + 1 ⋅ y + ( − 2 ) ⋅ z = x + y − 2 z = 0 n ⋅ v = − 1 ⋅ x + 2 ⋅ y + 0 ⋅ z = − x + 2 y = 0 n ⋅ w = − 1 ⋅ x + 0 ⋅ y + 1 ⋅ z = − x + z = 0 ⎩ ⎨ ⎧ 2 y + y − 2 ⋅ ( 2 y ) = 0 → − y = 0 x = 2 y z = x = 2 y ⟶ n = ( 0 , 0 , 0 )
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