I have these three expressions:
E1=(c1-c2)/(c1-c5)
E2=(c3-c4)/(c3-c7)
Es=(c1-c4)/(c1-c5)
How can I express Es in function of E1 and E2?
Es=E1+E2*(c3-c7)/(c1-c5)
Thank you for your time.
This is same type of problem you had in previous question and you were right - TI basic is not made for such things. There is no automatic way to do what you want but in each individual case you can do the same kind of gymnastics as previously. In this case, you have system of 2 equations with many variables, so you can eliminate 2 of them. You picked (for some reason) c2 and c4 to eliminate, so system must be solved for those 2 variables and solution included in third equation:
i1:=solve({E1=(c1-c2)/(c1-c5),E2=(c3-c4)/(c3-c7)},{c2,c4})
Es:=(c1-c4)/(c1-c5)|i1
result>
Es:=(c1+c3*(e2-1)-c7*e2)/(c1-c5)
Also, in this case E1 and E2 expressions are independent, so they can be solved separately, and Es expr. doesnt contain c2, you can just>
i1:=solve(E2=(c3-c4)/(c3-c7),c4)
Es:=(c1-c4)/(c1-c5)|i1
..with the same result. This is not what you wanted, but your solution is not correct anyway.