x²-70x+825=0x²-70x+1225-400=0(x-35)²-20²=0(x-35+20)(x-35-20)=0x-35+20=0x=15x-35-20=0x=55
解:因式分解:(X-55)(X-15)=0 (X-55)=0, X1=55, (X-15)=0, X2=15
825=5*5*3*11所以便可以十字相乘分解(x-15)(x-55)=0x1=15,x2=55
(x-35)²=400x-35=±20x1=55 x2=15
根据二元一次方程解的公式得x1=55 x2=15