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Q.

If a line makes 60o and 45o angles with the positive directions of  x-axis and z-axis  respectively, then find the angle that it makes with the positive direction of y-axis.  Hence, write the direction cosines of the line.

Or

Check whether the lines x12=y23=z34 and x45=y12=z are skew or not.

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answer is 1.

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Detailed Solution

Given: 𝛼 = 60𝑜 and 𝛾 = 450
Let angles α makes with x-axis, γmakes with z-axis and β-makes with y-axis then 
l=cos60,m=cosβ,n=cos45
We know that α2+β2+γ2=1
or cos260+cos2β+cos245=1
or 14+cos2β+12=1
or cos2β=11412=4124=14
or cosβ=±12
β=π3 or 2π3
From the data two lines lieing in  the the interval of 0,π2
Therefore, the angle that it makes with the positive direction of y-axis is β=π3=60
Now, for direction cossine we will use the following formula, 
If, l, m, n are the direction cosines, so, 
l=cosα=cos60=12m=cosβ=cos60=12n=cosγ=cos45=12
Therefore, the angle that it makes with the positive direction of y-axis is β=60 and the 3 direction cosines of the line is 12,12,12.

Or

Given: x12=y23=z34(1) and x45=y12=z01........(2)
So, x1=1,y1=2,z1=3 and x2=4,y2=1,z2=0 also l1=2,m1=3,n1=4 and l2=5,m2=2,n2=1
Now for to check the lines are skew then it should not be coplanar,
So, for coplanarity,
x2x1y2y1z2z1l1m1n1l2m2n2=0
Now, 
x2x1y2y1z2z1l1m1n1l2m2n2=411203234521313234521=3(38)(1)(220)3(415)=3(5)183(11)=1518+33=33+33=0
Therefore, we can say that lines are coplanar. Hence, given lines are not skew.

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