Given that cos42° is simpler to 0.743, What is the sine of the complementary angle?

Cos (42) = sin (90-42)

So in this case:

cos (42) = sin (48)

As:

A complementary angle is either of two angles whose sum is 90 degrees. {Source: Google}

The cos of an angle and the sine of its complement are the same number.

Spend a couple of minutes looking at a right triangle, and reviewing the definitions of sin and cos, and you'll see why.