Let G=GL(2,R) and let K be a subgroup of R*. Prove if H = {AeG | det(A)eK} show H is a subgroup of G. (Note: e means element of)
Here's my attempt. Let A,BeH then A,BeG so det(A),det(B)eK. Since K is a subgroup of R* then det(A)det(B)eK. Then since det(A)det(B)eK then ABeG and so ABeH.
Let BeH. Then BeG and det(B)eK. Since BeG then B^(-1)eG and so B^(-1)eH.
This doesn't seem right, so any help appreciated.