The length of a metal wire is l1 when the tension in it is T1 and is l2 when the tension is T2. The natural length of the wire is
Let the Area of cross-section of the wire be 'S' and it doesn't' vary as the length changes within the elastic limit.
In the first case, the elongation in the wire=l2−l
⇒Y1=(T1s)l(l1−l)
Now,the second case, the elongation in the wire =l2−l
⇒Y2=(T1s)l(l1−l)
Now, Y1=Y2 as the material is same
⇒T1ls(l1−l)=T2ls(l2−l)⇒T1l1−l=T2l2−lT1l2−T2l=T2l2−T2ll(T2−T1)=T2I2−T1I2l=T2l1−T1l2T2−T1