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)

in the options both c) and d) are given
But answer is c)
Why ?
:-(

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