If cos x = sin (20 + x ) degrees and 0 degrees < x < 90 degrees , the value of x is ___ degrees
cos x = sin (20 + x)
sin and cos are CO-FUNCTIONS, which means that: cos x = cos [90 - (20 + x)]
cos x = cos (90 - 20 - x)
cos x = cos (70 - x)
Therefore, x = 70 - x
x + x = 70
2x = 70