Jeśli α∈(0°,90°) i tgα=6/7 , to :
7sinα/(7sinα+cosα)=7sinα(1/sinα(7+cosα/sinα)=7/(7+ctgα)=7/((7+7/6)=7/(7 7/6)=7/(49/7)=7:49/7=7·7/49=49/49=1
ctgα=1/tgα ⇔ ctgα=1/(6/7) ⇔ ctgα=7/6
