1, according to the meaning of the question, build a coordinate system, where s is the origin, SA is the x axis, SB y, SC Z if a regular triangular pyramid has three sides 2a, then E(a, a, 0)F(0, 0, a)B(0, 2a, 0) and vector SE=(a, a). Vector FB=(0, 2a,-a); The cosine of the vector angle is (root sign 10)/5.

So the angle arccos ((root number 10)/5) formed by straight lines on different planes.

2. Draw the completed cube (side length 2a) inside, and make it into a plane triangle through translation, with three sides BF= root number 5 * a；; 2SE=S'D'=2* radical number 2 * a；; BD’= BF； The base angle of an isosceles triangle is arccos ((root number 10)/5).