一條數!!help!!please hep~

Extend BD to cut AC at M.
Therefore, angle BMC = angle BAM + angle ABM = 40 + 30 = 70 (exterior angle of triangle.)
Angle a = angle BMC + angle MCD = 70 + 25 = 95. (ext. angle of triangle).
So a is 95 degree.

2008-08-04 11:38:57 補充：
In general, whenever you see this figure, angle a will be equal the sum of these 3 angles. This is a very useful result.

延長BD
該點設為E點

∠CED=40+30=70 (△外角)
∵∠BDC= a
∴a=70+25=95 (△外角)