Write the value of .

Let

Expanding along R1,


we get ∆= sin20°cos70°-(-cos20°)sin70°


= sin20°cos70°+cos20°sin70°


Since sin(A+B)= sinAcosB+cosAsinB


Hence, sin20°cos70°+cos20°sin70°=sin(20°+70°)


=sin(90°)


=1


Hence, ∆=1


1