Advanced Trigonometry?

find cosx assuming it to be positive and place a negative after you computed the result

coz for obtuse angles cos is negative

to find cosx use the formula

sin(^2)x +cos(^2)x=1

(3/5)^2 + cos(^2)x=1

cos x= 4/5 or -4/5

the ans is -4/5 here as cos is negative for obtuse angles

If x is an obtuse angle, then cosx < 0

cos²x = 1 - sin²x and cosx < 0, therefore

cosx = -√(1 - sin²x)

cosx = -√(1 - 9/25)

cosx = -√(16/25)

cosx = -4/5

sinx=3/5

sin^2x+cos^2x=1

cos^2x=1-(9/25)

cos^2x=16/25

cosx=4/5 , -4/5

now it is said that x is obtuse .hencex>90deg) so x is in the second quadrant.in the second quad sine is positive but cosine is negative

hence csx= -4/5

cos(x) = -√(1 - 9/25) = -4/5

sin2y = cos4y = cos(2x2y) = a million - 2(sin2y)^2 => 2(sin2y)^2 + sin2y - a million = 0 that is a quadratic in terms of sin2y. in case you dont see this then permit sin2y=a, then 2a^2 + a -a million =0 (2a + a million)(a - a million) = 0 a = -0.5 or a = a million. subsequently: sin2y = -0.5 or sin2y = a million you should be waiting to unravel that your self, i'm no longer likely to do all your artwork.