平方原式得x^2+25y^2+10xy=36
观察与所求式间的差别,可将原式化为:x^2+5xy+(5xy+25y^2)=36
而5xy+25y^2=5y（x+5y)=30y,所以原式平方所得的x^2+25y^2+10xy=x^2+5xy+30y=36