In this way, we take the midpoint G of SB and connect EG and FG, so that EG is parallel to AB, GF is parallel to SC, and EG=GF= 1.

According to the cosine theorem, the angle EGF= 120 degrees, and the included angle between SC and AB is 60 degrees.

Supplement: Cosine theorem generally applies to triangles, but RT triangles are unnecessary. The formula is

Cosine theorem is an important theorem to reveal the relationship between the angles of a triangle. It can be directly used to solve the third side of a triangle with known two sides and included angles, or the angles of three sides. If the cosine theorem is deformed and properly transferred to other knowledge, it will be more convenient and flexible to use.

For any triangle with three sides A, B and C, a triangle with A, B and C satisfies the property.

(Note: a*b and a*c are respectively A times B and A times C C..A 2, B 2 and C 2 are the squares of A, B and C. )

a^2=b^2+c^2-2*b*c*CosA

b^2=a^2+c^2-2*a*c*CosB

c^2=a^2+b^2-2*a*b*CosC

CosC=(a^2+b^2-c^2)/2ab

CosB=(a^2+c^2-b^2)/2ac

CosA=(c^2+b^2-a^2)/2bc