This is a pun on 'cosine' which is the trigonometric function that is equal to the ratio of the side adjacent to an acute angle (in a right-angled triangle) to the hypotenuse, but it's also being used because the alternative spelling 'co-sign' can mean sign (a loan or lease) jointly with another person in order to guarantee payment., which in this context is just signing one's name
' ' Cosign here , ' ' read the first instruction on the trigonometry placement exam .