1) 49^(-1/2)*(1/7)^(-2)*2^(-1)*(-2)^(-2) = 1/49^(1/2) * 7^2 * 1/2 * 1/(-2)^2 =
= 1/7*49*1/2*1/4 = 7/8
2) 2a^3*x^5/(3b^2*y^4) * 6ay^3/(5bx^4) * by/(a^2*x^2) =
= 12a^4*b*x^5*y^4 / (15a^2*b^3*x^6*y^4) = 4a^2/(5xb^2)
3) (m-n)/(√m-√n) - (√m^3-√n^3)/(m-n) =
= (√m+√n)(√m-√n)/(√m-√n) - (√m-√n)(m+√(mn)+n)/[(√m-√n)(√m+√n)] =
= √m + √n - (m+√(mn)+n)/(√m+√n) =
= [(√m+√n)^2 - (m+√(mn)+n)]/(√m+√n) =
= (m+2√(mn)+n-m-√(mn)-n)/(√m+√n) = √(mn)/(√m+√n)