It d be the length of line, then projection on X-axis =dl, projection on Y-axis =dm and projection on Z-axis =dn Given, d=l and dl=I1 dm=l2 and dn=I3 Now, (dl)2+(dm)2+(dn)2=l12+l22+l32 d2(l2+m2+n2)=l12+l22+l32{∵12+m2+n2=1} l2.l=l12+l22+l32 ⇒ l12+l22+l32=l2