The value of sin 20° cos 70° + sin 70° cos 20° is :

sin20° cos 70° + sin70° cos 20°

We know,

sinA = cosB  Iff A + B = 90°

Similarly sin20° = cos70°

So,

= sin20° .sin 20° + cos20°. cos 20°

= sin²20° + cos²20°

= 1                 { sin²θ + cos²θ = 1 }