急 ! F5 Solving Triangles ex3

(1)

the largest angle should be B
let a = 4k, b = 5k, c = 2k

cos B = [(4k)^2 + (2k)^2 - (5k)^2] / [2(4k)(2k)]
cos B = -5k^2 / 16k^2
cos B = -5 / 16
B = 108.2


(4)

D = 180 - B
AC^2 = 4^2 + 6^2 - 2(4)(6)cos B => cosB = (52 - AC^2) / 48
AC^2 = 9^2 + 11^2 - 2(9)(11)cos(180 - B) => cosB = (AC^2 - 202) / 198

So, (52 - AC^2) / 48 = (AC^2 - 202) / 198
(52 - AC^2) / 48 = (AC^2 - 202) / 198
1716 - 33AC^2 = 8AC^2 - 1616
41AC^2 = 3332
AC = 9.01 cm