find the distance d (P1 P2) between the P1 and P2
P1=(-1,1);P2=(1,2)
d = sqrt [ ( x2 - x1 ) ^ 2 + ( y2 - y1 ) ^ 2 ]
In this case :
x1 = - 1
y1 = 1
x2 = 1
y2 = 2
d = sqrt [ ( 1 - ( - 1 ) ) ^ 2 + ( 2 - 1 ) ^ 2 ]
d = sqrt [ ( 1 + 1 ) ) ^ 2 + 1 ^ 2 ]
d = sqrt [ 2 ^ 2 + 1 ]
d = sqrt ( 4 + 1 )
d = sqrt ( 5 )