Look at the figure shown below: which step should be used to prove that point a is equidistant from points c and b?

Combine points A with C and A with B. Consider ΔABD and ΔACD:

1. AD is common side, then  AD\cong AD;

2.   CD\cong BD  - given in the diagram;

3.  \angle ADB\cong \angle ADC .

By SAS Postulate,  \triangle ABD\cong \triangle ACD . Congruent triangles have congruent corresponding sides and congruent corresponding angles, so  AC\cong AB .

From this proof you can see that correct choice is option D (In triangles ABD and ACD, two sides and an included angle are equal.)

Step-by-step explanation:

a_n = a_1 + (n - 1)d

the first term is 6.

the common difference d = ?

when you find d, replace n with 7 and d with the common difference to find your answer.