Question

The straight line with equation y=3/4x makes an acute angle theta with the x-axis.

Answer 1

The angle is arctan(3/4) => sin(2t) = sin(2arctan(3/4)) =
2sin(arctan(3/4))cos(arctan(3/4)) 

Let z = arctan(3/4) => tan(z) = 3/4 

2sin(arctan(3/4))cos(arctan(3/4)) = 2sin(z)cos(z) = 2(3/5)(4/5) = 24/25 

cos(2t) = cos^2(t) - sin^2(t) = cos^2(z) - sin^2(z) = (4/5)^2 - (3/5)^2 = (16 - 9)/25

= 7/25

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