Cos(pi) = -1
You should have this memorized along with other values such as
sin(0) = 0
cos(0) = 1
sin(pi/2) = 1
cos(pi/2) = 0
sin(pi) = 0
cos(pi) = -1
sin(3pi/2) = -1
cos(3pi/2) = 0
and remember that tan = sin/cos
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Because all are a multiple of 2, you can factor out 2, 2(-x^2+x+2)=0