In the right triangle ABC, the AB square AC square is why the BC square

Pythagorean theorem: A squared + B squared = C squared.

Use the cosine theorem: cosa (b squared c square a square yanling) divided by 2bc

It is also known that a square b square bc c square is substituted into the above Tongxiang pin formula to get it: cosa = -1 2, and the angle a is the inner angle of the triangle abc, so a is 120 degrees.

It is known from the cosine theorem.

a²=b²+c²-2bccosa

a²=b²+c²+bc

The two types are more auspicious than the destruction of the banquet sedan chair.

2bccosa =bc

2cosa=1

cosa=-1/2

a=2 3,5, according to the cosine fibre.

cosa=（b²+c²-a²）/2bc）

And a = b + c + bc, substituting the above formula, that is, obtain.

cosa = (b +c -(b +c +bc)) 2bc)=-1 2 so angle a = 120°,0,120 degrees,0,

Multiply the wild potato search 2 on both sides of the equation at the same time, and then move the right one to the left, you can get (a-b) 2+(a-c) 2+(b-c) 2=0, that is, a=b=c, so it is an equal hand missing edge triangle.

A 2+b 2+c 2=ab+ac+bca 2+b 2+c 2-ab-ac-bc=02a 2+2b 2+2c 2-2ab-2ac-2bc=0 with Zheng Daifang: (a-b) 2+(a-c) 2+(b-c) 2=0 can only be a-b=0, a-c=0, b-c=0, b-c=0

So a=b=c

The positive and negative value of the cosine value can be considered through the cosine theorem, and then judged, the principle is as follows:

Cosine theorem: cosa=(b*b+c*c-a*a) (2*b*c) If the sum of the squares of the two smaller sides is greater than the third side, it is an acute triangle, if the sum of the squares of the smaller sides is less than the third side, it is an obtuse triangle, and if the sum of the smaller two sides is greater than the third side, it is a right triangle.

1) is an acute triangle, 2) is an obtuse triangle.

3）b-a2

Party A + Party B + Party AB-C = 0

a²+b²-c²=-ab

So cosc=(a +b -c) 2ab=-ab 2ab=-1 2 so c = 120 degrees.

The square of a, the square of b, the square of c, ab+ac+bc, the square of 2a, the square of 2b, the square of 2c, the square of 2ab+2ac+2bc, the square of 2a, the square of 2b, the square of 2c, the square of 2c, the square of 2ab-2ac-2bc=0 (the square of a-2ab the square of b) + (the square of b - the square of 2bc c) + (the square of a - the square of 2ac c) = 0

a-b) squared + (b-c) squared + (a-c) squared = 0 a-b = 0, b-c = 0, a-c = 0

A = b = c triangle is an equilateral triangle.

By the cosine theorem a 2 = b 2 + c 2 - 2bc * cosa so -2 cosa = 1

cosa=-1/2

a = 120 degrees.