We would prove this with a right isosceles triangle. The diagram is shown below
Cosidering triangle ABC, if we take angle A as the reference angle,
opposite side = BC = 1
adjacent side = AB = 1
hypotenuse = AC
The tangent trigonometric ratio is expressed as
Tan # = opposite side/adjacent side
Thus,
Tan 45 = 1/1
Tan 45 = 1
