cosA=√(1-sin2A)=√(1-0.2800)2=√(1-0.0784)=√0.9216=0.9600.
tanA=sinA/cosA=0.2800/0.9600=7/24 or 0.2917 approx.
Another way: sinA=0.28=7/25=opposite/hypotenuse.
adjacent2+opposite2=hypotenuse2 (Pythagoras).
adjacent2=hypotenuse2-opposite2=252-72=(25-7)(25+7)=18×32=9×64.
adjacent=√(9×64)=3×8=24.
cosA=adjacent/hypotenuse=24/25; tanA=opposite/adjacent=7/24.