1/√2
Explanation:
Imagine a right-angled triangle, for e.g. Triangle ABC,
Where
B = 90°
Remember your Toa Cah Soh.
sin 45° = opposite / hypotenuse
If theta = 45°
Angles A and C = 45° (Sum of angles in a triangle equal = 180°)
Since theta are the same, we can also deduce that the length AB and BC are the same.
Take for example AB = BC = 1 = opposite
By the Pythagorean theorem,
AB² + BC² = AC²
1² + 1² = AC²
2 = AC²
AC = √2 = hypotenuse
Therefore:
sin 45° = opposite / hypotenuse
sin 45° = 1/√2