Trigonometric equation for stress

[foo9008]

Homework Statement

is the circled part wrong ?

Homework Equations

The Attempt at a Solution

how could 0.5 sin2theta = ( 1 + cos2theta ) / 2 ?

 

[SteamKing]

Homework Statement

is the circled part wrong ?

Homework Equations

The Attempt at a Solution

how could 0.5 sin2theta = ( 1 + cos2theta ) / 2 ?

It's a trig identity. Look up "Half-Angle" formulas:

https://en.wikipedia.org/wiki/List_of_trigonometric_identities