a+b=45

--->tan(a+b)=tan45`

--->(tana+tanb)/(1-tanatanb)=1

--->tana+tanb=1-tanatanb

--->tanatanb+tana+tanb+1=2

--->(1+tana)(1+tanb)=2

21+24=45,22+23=45

--->(1+tan21)(1+tan4)=2,(tan22)(1+tan23)=2

--->(1+tan21)(1+tan22)(1+tan23)(1+tan24)=4.