What is the final step to prove that Sn 1 + 2 + 3 + ... + n = n(n+1)/2? a. Verify that Sn is valid for n = 1 c. Assume that Sn is valid for n = k b. Show that Sn is valid for n = k d. Prove that Sn is valid for n = k + 1

The pyraminx is a rubik's cube-type toy in the shape of a tetrahedron. the pyraminx shown below has edges 15\,\text{cm}15cm long and vertical height h=12.2\,\text{cm}h=12.2cm. the triangle drawn with dashed lines is a right triangle. what is the distance rr? round your answer to the nearest tenth.