y=sin(x+2)sinx−sin2(x+1)
=21[2sin(x+2)sinx−2sin2(x+1)]
[\because 2\mathrm{sin}A\mathrm{sin}B =\mathrm{cos}(A-B)-\mathrm{cos}(A+B)&\mathrm{cos}2A=1-2{\mathrm{sin}}^{2}A]
=21[cos(x+2−x)−cos(2x+2)−(1−cos2(x+1))]
=21[cos2−cos(2x+2)−1+cos(2x+2)]
=21[1−cos2]=−212sin21
=−sin21( constant and negative )
Hence, the straight line lies in the third and fourth quadrants only.