試解

Extend DE to F. Obviously, ABFD=21*2=42, which means ADC=21, which means ADE=21-12=9, which means BFE=21-9=12.

Let EFC area =x. Observe that BFE and EFC share the same bottom line, while EFC and ADE are similar. So, 12/x= BF/FC=AE/EC= sqrt(9/x)

x=144/9=16

Whole area = 42+12+x=70

 

所有跟帖： 

• great, I got the same answer in a more complex way -皋誠- ♀ (45784 bytes) () 03/20/2026 postreply 18:43:30

• update -皋誠- ♀ (19240 bytes) () 03/20/2026 postreply 19:02:55

• 漂亮，你提供的視角讓我也學習到了，最後的解法簡單得驚人 -monseigneur- ♂ (0 bytes) () 03/20/2026 postreply 22:05:43