(15-x)(15-x)=25x
225-30x＋x²=25x
x²-55x+225=0
x=(-b±√b²-4ac)/2a
=(55±√55²-4×1×225)/2
=55±√3025-900)/2
=(55±√2125)/2
解得：x1=(55+5√85）／2 x2=（55-5√85)/2
(15-x)(15-x)=25x
225-30x＋x²=25x
x²-55x+225=0
x=(-b±√b²-4ac)/2a
=(55±√55²-4×1×225)/2
=55±√3025-900)/2
=(55±√2125)/2
解得：x1=(55+5√85）／2 x2=（55-5√85)/2