15)
a+b=180°
a=50°+b
50°+b+b=180°⇒2b=180°-50°⇒2b=130°⇒b = 65°
a = 50° + 65° = 115° deci a = 115°
16)
a - b = 20°
a + b=180°
adunam cele 2 relatii
2a= 20 + 180°⇒ 2a = 200⇒ a = 100°
dar a+b=180°⇒ b=180° - a = 180° -100° = 80°
deci a = 100° si b = 80°
17) a+b=180°
b=(1/3)×a=a/3
a+a/3=180°⇒4a=540°⇒a=540:4
a=135°
b=a/3=135/3=45°
deci a =135° si b = 45°
