Find the value

(a) 2

(b) -2

(c) xy + 1/xy

(d) x/y + y/x

Take your pick, any of the above are valid under different conditions (implying that you ought to have mentioned them)

2cosαcosβ-2sinαsinβ=?

x=cosα±√cos²Î±-11

y=cosβ±√cos²Î²-11

u shall check
Is x²y²+1=x²+y²?????

It is not So
So book is incorrect[1]

above was regarding to #3