Heat on rod!

Two straight metallic strips each of thickness $t$ and length $l$ are riveted together. Their coefficients of linear expansion are $\alpha_1$ and $\alpha_2$. If they are heated through a temperature of $\Delta t$, the bimetallic strip will bend to form an arc of radius $r.$

Find $r.$

• (a) $\ \dfrac t{(\alpha_1 + \alpha_2) \Delta t}$
• (b) $\ \dfrac t{(\alpha_2-\alpha_1) \Delta t}$
• (c) $\ {(\alpha_1+\alpha_2) \Delta t}$
• (d) $\ {(\alpha_2-\alpha_1) \Delta t}$

Type 1 for option (a), 2 for option (b), 3 for option (c), or 4 for option (d).