15＋3根号35/2
x分之y+y分之x=（x²+y²)/xy=[(x+y)²-2xy]/xy=(x+y)²/xy-2=[¼(√7+√5+√7-√5)²]／[¼﹙√7+√5)(√7-√5)]-2=7÷½-2=14-2=12
