if you graph y=\sqrt{x}
and you see at x = 0
those kind of points are considered critical points, because the function 'ends' there
any 'sharp' points ( like cusps) or single points are critical points
more egs
y=\left | x \right |\\\\ y=\sin^{-1}x
the sharp point on y = absx and the points for...
that is better and you can shorten it
\\assume\\\\ 7^k(1+3k)-1=9M \\\\for\ n = k+1\\\\ 7^{k+1}+3(k+1)(7^{k+1})-1\\\\ =7^{k+1}(3k+4)-1\\ =7^k(21k+28)-1\\ =7^k[(1+3k)+(27+18k)]-1\\ =7^k(1+3k)-1+7^k(9)(3+2k)\\ =9M+9(7^k)(3+2k)\\\\ which\ is\ divisible\ by\ 9
they are saying that CA = wCB, which is wrong
eg
if you have p=iq, then p =cis(pi/2)q so you are rotating vector q 90 deg anti clockwise
so it should be CB = wCA