50-2a²=2(25-a²)=2(5+a)(5-a)
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-24m^2-24n-16m-36mn=(-24m^2-16m)+(-24n-36mn)= -8m(3m+2)-12n(3m+2)= (3m+2)(-8m-12n)
Vср=S/t=(S1+S2+S3)/(t1+t2+t3) =(300+300+300)/(S1/v1+S2/v2+S3/v3)=900/(300:60+300:100+300:75)=900/(5+3+4)=900/12=75км/ч
Ответ: 75км/ч.