Main/central vertex

ABC is an isosceles triangle with base BC and main vertex A. The angle at vertex A is 18°. What will be the size of the angle at vertex B?

Correct answer:

B =  81 °

Step-by-step explanation:

$$\begin{gathered} A=18\text{ ° } \\ C=B \\ A+B+C=180\mathrm{°} \\ A+B+B=180 \\ 18+B+B=180 \\ 2B=162 \\ B=81=81\text{°} \end{gathered}$$

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