the vertex angle of an isosceles triangle measures 40°. what is the measure of a base angle?\n40°\n70°\n100°\…

Explanation:

Step1: Recall angle - sum property of triangle

The sum of the interior angles of a triangle is $180^{\circ}$. In an isosceles triangle, the two base - angles are equal. Let the measure of each base - angle be $x$.

Step2: Set up an equation

We know that the vertex angle is $40^{\circ}$ and the sum of the three angles of the triangle is $180^{\circ}$. So, $40 + x+x=180$.

Step3: Simplify the equation

Combining like terms, we get $40 + 2x=180$.

Step4: Solve for $x$

Subtract 40 from both sides: $2x=180 - 40=140$. Then divide both sides by 2: $x=\frac{140}{2}=70$.

Answer:

B. $70^{\circ}$