sec^(-1)xsec−1x is defined for xin(R-]-1,1[)rarr([0,pi]-{pi/2})∈(R−]−1,1[)→([0,π]−{π2}). sec^(-1)secx= xsec−1secx=x if and only if xin([0,pi]-{pi/2})∈([0,π]−{π2}).
in your question, since (7pi)/10in([0,pi]-{pi/2})7π10∈([0,π]−{π2})
so, sec^(-1)sec((7pi)/10)=(7pi)/10sec−1sec(7π10)=7π10