First off we want to note that sin(x) = cos(90-x)
Then if you plug in 17 for x you get:
sin(17-5) = cos(4*17+10)
which is
sin(12) = cos(78)
now lets see if this equation is equal by using sin(x) = cos(90-x)
so x would be 12 in this case.
so
sin(12) = cos(90-12)
sin(12) = cos(78)
this is the same answer that we got before (in bold)
so the statement is true
