If it has rational coefients and is a polygon
if a+bi is a root then a-bi is also a root
the roots are -4 and 2+i
so then 2-i must also be a root
if the rots of a poly are r1 and r2 then the factors are
f(x)=(x-r1)(x-r2)
roots are -4 and 2+i and 2-i
f(x)=(x-(-4))(x-(2+i))(x-(2-i))
f(x)=(x+4)(x-2-i)(x-2+i)
expand
f(x)=x³-11x+20
It looks like you are trying to factor a quadratic. So you are looking for the numbers that when you multiply them, you get the top number (100), but when you add them you get the bottom number (-20).
Those two numbers are -10 and -10.
Hope that helps!
u = 9 i - 6 j
v = -3 i - 2j
w = 19 i + 15 j
u • v = (9 i - 6 j) • (-3 i - 2j)
Distribute the dot products:
u • v = 9*(-3) (i • i) + 9*(-2) (i • j) + (-6)*(-3) (j • i) + (-6)*(-2) (j • j)
i and j are orthogonal unit vectors, so their dot products are 0, while i • i = j • j = 1. So we have
u • v = 9*(-3) + (-6)*(-2) = -27 + 12 = -15
In other words, the dot product can be computed by simply multiplying corresponding components, and taking the total.
u • w = 9*19 + (-6)*15 = 81