【增效提能演练】
1 - 3 A C A
4、58° 50° 98°
5、解:设∠B = x°,则∠C = 5x°.
因为∠A + ∠B + ∠C = 180°,
所以60 + x + 5x = 180,解得x = 20,即∠B = 20°.
6、B
7、100°
8、解:(1)130°
(2)122°
(3)128°
(4)90°+ 1/2m°
(5)60°
(6)∠B0C = 90°+ 1/2∠A.
理由:因为BO,CO分别平分∠ABC和∠ACB,
所以∠OBC = 1/2∠ABC,∠OCB = 1/2∠ACB.
所以∠B0C = 180°- (∠0BC + ∠0CB)
= 180°-(1/2∠ABC + 1/2∠ACB)
= 180°- 1/2(180°- ∠A)
= 180° - 90°+ 1/2∠A
= 90° + 1/2∠A.
9、解:因为MN方向是南偏东30°,MA方向是南偏东60°,
所以∠AMB = 60° - 30° = 30°.
因为BA方向为南偏东75°,
所以∠ABN = 75° - 30°= 45°,
所以∠MBA = 180° - 45° = 135°.
在△MAB中,∠A = 180° - 30° - 135° = 15°