Cho a>b , c>0 thoa man : a^2 + b^2 + c^2 = ( a+b)^2 + (b-c)^2 + (c+a)^2 va bc – ab – ca = 25
Tinh a-b-c =0
Cho a>b , c>0 thoa man : a^2 + b^2 + c^2 = ( a+b)^2 + (b-c)^2 + (c+a)^2 va bc – ab – ca = 25 Tinh a-b-c =0
By Quinn
By Quinn
Cho a>b , c>0 thoa man : a^2 + b^2 + c^2 = ( a+b)^2 + (b-c)^2 + (c+a)^2 va bc – ab – ca = 25
Tinh a-b-c =0
Đáp án:
\(\pm 10\)
Giải thích các bước giải:
Ta có: \(a^2+b^2+c^2=(a+b)^2+(b-c)^2+(c+a)^2\)
\(\to a^2+b^2+c^2=a^2+b^2+2ab+b^2+c^2-2bc+a^2+c^2+2ac\)
\(\to a^2+b^2+c^2+2ab+2ac-2bc=0\)
\(\to a^2+b^2+c^2-2(bc-ab-ca)=0\)
\(\to a^2+b^2+c^2-2\cdot 25=0\)
\(\to a^2+b^2+c^2=50\)
Ta có: \((a-b-c)^2=a^2+b^2+c^2-2ab+2bc-2ac\)
\(\to (a-b-c)^2=a^2+b^2+c^2-2ab+2bc-2ac\)
\(\to (a-b-c)^2=a^2+b^2+c^2+2(bc-ab-ac)\)
\(\to (a-b-c)^2=50+2\cdot 25=100\)
\(\to a-b-c=\pm 10\)