math
posted by HongKit .
A vector equation for a given straight line is r = (i + 3j) + lambda (i  j)
Construct a vector equation for the line that does go through the point (1,2), and is perpendicular to r.
and
Determine the point of intersection of the two lines

you know that ij is perpendicular to ij since (1)(1) + (1)(1) = 1+1 = 0
so, i+2j + alpha(i  j) will do the trick
1lambda = 1 + alpha
3lambda = 2  alpha
intersect where lambda = 1/2 alpha = 1/2, at the point (1/2,5/2)