Câu 1:
A=2a2b2+2b2c2+2a2c2−a4−b4−c4" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">A=2a2b2+2b2c2+2a2c2−a4−b4−c4A=2a2b2+2b2c2+2a2c2−a4−b4−c4
⟺A=4a2c2−(a4+b4+c4−2a2b2+2a2c2−2b2c2)" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">⟺A=4a2c2−(a4+b4+c4−2a2b2+2a2c2−2b2c2)⟺A=4a2c2−(a4+b4+c4−2a2b2+2a2c2−2b2c2)
⟺A=4a2c2−(a2−b2+c2)2" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">⟺A=4a2c2−(a2−b2+c2)2⟺A=4a2c2−(a2−b2+c2)2
⟺A=(2ac+a2−b2+c2)(2ac−a2+b2−c2)" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">⟺A=(2ac+a2−b2+c2)(2ac−a2+b2−c2)⟺A=(2ac+a2−b2+c2)(2ac−a2+b2−c2)
⟺A=((a+c)2−b2)(b2−(a−c)2)" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">⟺A=((a+c)2−b2)(b2−(a−c)2)⟺A=((a+c)2−b2)(b2−(a−c)2)
⟺A=(a+b+c)(a+c−b)(b+a−c)(b−a+c)" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">⟺A=(a+b+c)(a+c−b)(b+a−c)(b−a+c)⟺A=(a+b+c)(a+c−b)(b+a−c)(b−a+c)
Mà a, b, c" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">a, b, ca, b, c là 3" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">33 cạnh của tam giác nên:{a+b+c>0a+c−b>0b+a−c>0b−a+c>0⟹(a+b+c)(a+c−b)(b+a−c)(b−a+c)>0" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">⎧⎩⎨⎪⎪⎪⎪a+b+c>0a+c−b>0b+a−c>0b−a+c>0⟹(a+b+c)(a+c−b)(b+a−c)(b−a+c)>0{a+b+c>0a+c−b>0b+a−c>0b−a+c>0⟹(a+b+c)(a+c−b)(b+a−c)(b−a+c)>0
⟹A>0 (Dpcm)" role="presentation" style="font-family: "Open Sans", sans-serif; display: inline; line-height: normal; font-size: 15px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">⟹A>0 (Dpcm)