the question begin with prove that
$(\sin A+\cos A)^2 +(\sin B+\cos B)^2+(\sin C+\cos C)^2=\sin 2A-\sin 2B-\sin 2C-1$
subsequently
Given $A+B+C = 180^\circ$ it is proved that $(\sin A+\cos A)^2 +(\sin B+\cos B)^2+(\sin C+\cos C)^2=-4\sin A \cos B \cos C-1$
Last, it asked us to find the range of $\cos B\cos C$ if $A=90^\circ$