what do they mean by 1 over sine 0
1/0 is not defined since if a*b = 0 , over a field , a = 0 , or b = 0 . but if 1/0 is defined , 0 * a = 1 => a = 1/0 , but a isnt possible , since 0 * m = 0:
a * 0 = a(0+0)
a * 0 = a*0 + a*0 / -(a*0)
0 = a*0
