>>775
(1)
AD↑ = (1/2) AB↑ + (1/2)AC↑
AE↑ = (1/2)AD↑ = (1/4)AB↑+ (1/4)AC↑

xAP↑+2BP↑+CP↑=0
xAP↑+2(AP↑-AB↑)+AP↑-AC↑=0
(x+3)AP↑ -2AB↑-AC↑ = 0
AP↑ = {2/(x+3)}AB↑ + {1/(x+3)}AC↑

AP↑ = {2/(x+3)}AB↑ + {1/(x+3)} (4AE↑-AB↑)
= { 1/(x+3)} AB↑ + { 4/(x+3) } AE↑
BE上にPがあるといことは係数の和が1だから
{ 1/(x+3)} + { 4/(x+3) } = 5/(x+3) = 1
x = 2