The answer on page 142 of the second volume of the seventh grade mathematics textbook is: the top angle of an isosceles triangle is 120 degrees, and the bottom height is higher than 3cm, so what is the waist length? ... very simple

... because the top angle of the isosceles mountain is 120 degrees ... so the remaining two feet are 30 degrees respectively. ..

The height of the low point is 3 cm (that is, the height of 30 degrees) ... Because the height consists of two right-angled triangles ... so the hypotenuse is the waist length of the isosceles triangle ... so the waist length is twice that of the right angle (in the right-angled triangle, the right-angled side of 30 degrees is half of the hypotenuse ...), so it is equal to 6.

... very clear ...

Take it.