Angle at the apex

In an isosceles triangle, the angle at the apex is 30° greater than the angle at the base. How big are the internal angles?

Correct answer:

A =  50
B =  50
C =  80

Step-by-step explanation:


A+B+C = 180
A=B
C = 30+A

A+B+C = 180
A-B = 0
A-C = -30

Row 2 - Row 1 → Row 2
A+B+C = 180
-2B-C = -180
A-C = -30

Row 3 - Row 1 → Row 3
A+B+C = 180
-2B-C = -180
-B-2C = -210

Row 3 - -1/-2 · Row 2 → Row 3
A+B+C = 180
-2B-C = -180
-1.5C = -120


C = -120/-1.5 = 80
B = -180+C/-2 = -180+80/-2 = 50
A = 180-B-C = 180-50-80 = 50

A = 50
B = 50
C = 80

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