When two forces F1 and F2 act on the same object, the resultant force is F. the angle between F1 and F2 is r F²=F1²+F2²+2×F1×F2×cosr

When two forces F1 and F2 act on the same object, the resultant force is F. the angle between F1 and F2 is r F²=F1²+F2²+2×F1×F2×cosr

Because the size of DAB is r, then the size of ABC is 180-r
AC²=AB²+BC²-2×AB×BC×cos∠ABC
That is: F & # 178; = F1 & # 178; + F2 & # 178; - 2 × F1 × F2 × cos (180-r)
Because & nbsp; & nbsp; cos (180-r) & nbsp; = & nbsp; - cosr
Therefore, F & # 178; = F1 & # 178; + F2 & # 178; + 2 × F1 × F2 × cosr